3R  YOUNG  FOL 


ELM 


ASTRONOMY 
FOR  YOUNG  FOLKS 


NORTHERN  PORTION  OF  THE  MOON  AT  LAST  QUARTER 
Taken  with  loo-inch  Hooker  Telescope  of  the  Mt.  Wilson  Observatory 

(See  Chapter  XXI) 


ASTRONOMY 

for 

YOUNG  FOLKS 


BY 


ISABEL    MARTIN    LEWIS,  A.  M. 

(Connected  with   the   Nautical   Almanac   Office 
of  the  U.  S.  Naval  Observatory) 


NEW  YORK 
DUFFIELD  AND  COMPANY 

1922 


Copyright,  1921,  by 
THE  CENTURY  COMPANY 

Copyright,  1922,  by 

DUFFIELD  AND  COMPANY 


Printed  in  U.  S.  A. 


CONTENTS 

CHAPTER  PAGE 

Preface       ......      ,     . .  «    xiii 

I.  The   Constellations       ......       3 

II.  January       .      ....      .      .      .      .  .     15 

III.  February     .      .      ...      .      .      .  .     21 

IV.  March    .      .      .      .      .      .      .      .      .  .     28 

V.  April  .  ...  v  .-""^  '  *  *  35 

VI.  May  .  .  .  .  f  .  .  .  .  .  41 

VII.  June       ;/.    .      ......      .  .     49 

VIII.  July       .  ;  .      .      .      .      .      .      .-..-.     56 

IX.  August        ,     .      .      .      .      .      .      ,  .     64 

X.  September    .     -,      ...      .      .      .  .     71 

XL  October  .  .  .  .  ,  .  .  .  .  78 

XII.  November    .      ,     ....      .      .  .     84 

XIII.  December    . .     90 

XIV.  Stars  of  the  Southern  Hemisphere     .  .     96 
XV.  The  Milky  Way  or  Galaxy     .      .      .  .107 

XVI.  The  Surface  of  the  Sun    .      .      .      .  .113 

XVII.  The  Solar  System       ...      .'     .  .119 

XVIII.  The  Origin  of  the  Earth  .      .      .      ,  .   127 

XIX.  Jupiter  and  His  Nine  Moons       .      .  .139 

XX.  The  Rings  and  Moons  of  Saturn      .  .    148 

XXI.  Is  the  Moon  a  Dead  World 156 

XXII.  Comets        .     .     .     .      .      .      .     V  .   165 

XXIII.  Meteorites        ........    173 

V 


493929 


vi  CONTENTS 

CHAPTER  PAGE 

XXIV.  The  Earth  As  a  Magnet 183 

XXV.  Some  Effects  of  the  Earth's  Atmosphere 

Upon  Sunlight   . 193 

XXVI.  Keeping  Track  of  the  Moon  ....   207 
XXVII.  The  Motions  of  the  Heavenly  Bodies     .   216 
XXVIII.  The  Evolution  of  the  Stars— From  Red 

Giants  to  Red  Dwarfs 225 

XXIX.  Double  and  Multiple  Stars       ....   230 

XXX.  Astronomical    Distances 241 

XXXI.  Some  Astronomical  Facts  Worth  Remem- 
bering       250 


ILLUSTRATIONS 


PAGE 

Northern  Portion  of  the  Moon  at  Last  Quarter 

Frontispiece 
The     Great     Hercules     Cluster — A    Universe     of 

Suns      .......      .  facing  page     56 

A  Dark  Nebula:  The  Dark  Bay  or  Dark  Horse 

Nebula  in  Orion facing  page  110 

A.    Venus.      B.    Mars.      C.    Jupiter.     D.    Saturn 

facing  page  122 

Spiral  Nebula  in  Canes  Venatici  .      .   facing  page  216 
Spiral    Nebula   in   Andromeda   Viewed   Edgewise 

facing  page  222 


Vll 


LIST  OF  TABLES 


PAGE 

I.  The  Principal  Elements  of  the  Solar  System  261 

II.  The  Satellites   of  the   Solar   System    ...   262 

III.  The  Twenty  Brightest  Stars  in  the  Heavens  .   263 

IV.  A  List  of  the  Principal  Constellations  .      .  264-265 

V.  Pronunciations   and   Meanings   of    Names   of 

Stars  and  Constellations  .  266-267 


IX 


PREFACE 

ASTRONOMY,  it  has  been  said,  is  the  oldest  and  the 
noblest  of  the  sciences.  Yet  it  is  one  of  the  few  sci- 
ences for  which  most  present-day  educators  seem  to  find 
little,  if  any,  room  in  their  curriculum  of  study  for 
the  young,  in  spite  of  its  high  cultural  value.  It  is, 
we  are  told,  too  abstruse  a  subject  for  the  youthful 
student.  This  is  doubtless  true  of  theoretical  or  mathe- 
matical astronomy  and  the  practical  astronomy  of  the 
navigator,  surveyor  and  engineer,  but  it  is  not  true  of 
general,  descriptive  astronomy.  There  are  many  dif- 
ferent aspects  of  this  many-sided  science,  and  some  of 
the  simplest  and  grandest  truths  of  astronomy  can  be 
igrasped  by  the  intelligent  child  of  twelve  or  fourteen 
years  of  age. 

Merely  as  a  branch  of  nature  study  the  child  should 
have  some  knowledge  of  the  sun,  moon,  stars  and 
planets,  their  motions  and  their  physical  features,  for 
they  are  as  truly  a  part  of  nature  as  are  the  birds, 
trees  and  flowers,  and  the  man,  woman  or  child  who 
goes  forth  beneath  the  star-lit  heavens  at  night  abso- 
lutely blind  to  the  wonders  and  beauties  of  the  uni- 
verse of  which  he  is  a  part,  loses  as  much  as  the  one 
who  walks  through  field  or  forest  with  no  thought  of 
the  beauties  of  nature  that  surround  him. 

The  astronomer  is  the  pioneer  and  explorer  of  to- 


xii  PREFACE 

day  in  realms  unknown  just  as  the  pioneers  and  ex- 
plorers of  several  centuries  ago  were  to  some  extent 
astronomers  as  they  sailed  unknown  seas  and  traversed 
unexplored  regions.  As  the  years  pass  by  the  astron- 
omer extends  more  and  more  his  explorations  of  the 
universe  and  brings  back  among  the  fruits  of  dis- 
covery measures  of  giant  suns  and  estimates  of  the 
form  and  extent  of  the  universe,  views  of  whirling, 
seething  nebulae,  mysterious  dark  clouds  drifting 
through  space,  tremendous  solar  upheavals  or  glimpses 
of  strangely  marked  surfaces  of  nearby  planets. 

In  the  following  pages  the  author  has  endeavored  to 
tell  in  words  not  beyond  the  comprehension  of  the 
average  fourteen-year-old  child  something  of  the  nature 
of  the  heavenly  bodies.  In  Part  I  an  effort  is  made  to 
make  the  child  familiar  with  the  stars  by  indicating  when 
and  where  they  can  be  found  in  the  early  evening  hours. 
In  addition  to  identifying  the  principal  constellations  and 
their  brightest  stars  by  means  of  diagrams  an  attempt 
has  been  made  to  acquaint  the  child  with  the  most  inter- 
esting recent  discoveries  that  have  been  made  concerning 
the  principal  stars  or  objects  in  each  group  as  well  as 
with  some  of  the  stones  and  legends  that  have  been  as- 
sociated with  these  groups  of  stars  for  centuries,  and 
that  have  been  handed  down  in  the  folk-lore  of  all  nations. 

Chapters  2-13,  inclusive,  appeared  originally  with 
diagrams  similar  to  those  shown  here,  under  the  depart- 
ment of  Nature  and  Science  for  Young  Folk  in  St. 
Nicholas  from  May,  1921,  to  April,  1922,  inclusive. 
The  Introductory  Chapter  and  Chapters  14  and  15, 


PREFACE  xiii 

on  the  Milky  Way  and  Stars  of  the  Southern  Hemi- 
sphere, respectively,  are  published  here  for  the  first  time, 
as  is  also  the  chapter  in  Part  II  on  the  Evolution  of  the 
Stars  from  Red  Giants  to  Red  Dwarfs,  which  gives  the 
order  of  the  evolution  of  the  stars  as  now  accepted  as  a 
result  of  the  brilliant  astronomical  researches  of  Dr. 
Henry  Norris  Russell  of  the  United  States  and  Prof.  A. 
S.  Eddington,  of  England. 

The  remaining  chapters  in  Part  II  have  been  chosen 
from  a  series  of  articles  that  have  appeared  in  Science 
and  Invention,  formerly  The  Electrical  Experimenter, 
in  the  past  four  years,  and  have  been  considerably 
revised  and  in  some  parts  rewritten  to  adapt  them  to  the 
understanding  of  more  youthful  readers.  These  chapters 
deal  with  a  variety  of  astronomical  subjects  of  gen- 
eral popular  interest  and  an  effort  has  been  made  to 
select  subjects  that  would  cover  as  wide  an  astronomical 
field  as  possible  in  a  limited  space. 

The  author's  aim  has  not  been  to  write  a  text-book 
of  astronomy  or  to  treat  in  detail  of  any  one  aspect  of 
this  extensive  science,  but  simply  to  give  the  average 
child  some  general  knowledge  of  the  nature  of  the  heav- 
enly bodies,  both  those  that  form  a  part  of  our  own 
solar  system  and  those  that  lie  in  the  depths  of  space 
beyond. 

It  has  been  necessary  to  write  very  briefly,  and  we 
feel  inadequately,  of  many  topics  of  special  interest 
such  as  the  sun  and  moon.  Books  have  been  written 
on  these  two  subjects  alone  as  well  as  upon  such  sub- 
jects as  Mars,  eclipses,  comets,  meteors,  etc.,  but  the 


XIV 


PREFACE 


object  has  been  to  acquaint  the  child  with  the  out- 
standing features  of  a  variety  of  celestial  objects  rather 
than  to  treat  of  a  few  in  detail. 

If  the  writer  succeeds  in  arousing  the  child's  inter- 
est in  the  stars  so  that  he  may  look  forth  with  intelli- 
gence at  the  heavens  and  greet  the  stars  as  friends  and 
at  the  same  time  grasps  some  of  the  simplest  and  most 
fundamental  of  astronomical  truths  such  as  the  dis- 
tinction between  stars  and  planets,  the  motions  of  the 
heavenly  bodies  and  their  relative  distances  from  us 
and  the  place  of  our  own  planet- world  in  the  universe, 
this  book  will  have  served  its  purpose. 


ASTRONOMY 
FOR  YOUNG  FOLKS 


"The  heavens  declare  the  glory  of 
God,  and  the  firmament  showeth 
His  handiwork." 

Psalm  XIX. 


Astronomy  for  Young  Folks 


THE  CONSTELLATIONS 

"Canst  thou  bind  the  sweet  influences  of  the  Pleiades 
Or  loose  the  bands  of  Orion  ? 

thou  bring  forth    lazzaroth  in    Is  season 
Or  canst  thou  guide  Arcturus  with  his  sons?" 

— BOOK  OF  JOB. 

WHO  would  not  like  to  know  the  stars  and  constella- 
tions by  their  names  and  in  their  seasons  as  we 
know  the  birds  and  the  trees  and  the  flowers,  to  recognize 
at  their  return,  year  by  year,  Sirius  and  Spica,  Arcturus 
and  Antares,  Vega  and  Altair,  to  know  when  Ursa 
Major  swings  high  overhead  and  Orion  sinks  to  rest  be- 
neath the  western  horizon,  when  Leo  comes  into  view 
in  the  east  or  the  Northern  Crown  lies  overhead? 

Often  we  deprive  ourselves  of  the  pleasure  of  making 
friends  with  the  stars  and  sfiut  our  eyes  to  the  glories 
of  the  heavens  above  because  we  do  not  realize  how 
simple  a  matter  it  is  to  become  acquainted  with  the 
various  groups  of  stars  as  they  cross  our  meridian, 
one  by  one,  day  after  day  and  month  after  month  in 

3 


,  ;<   t.   !  ASTRONOMY  FOR  YOUNG  FOLKS 

the  same  orderly  sequence.  When  the  robin  returns 
once  more  to  nest  in  the  same  orchard  in  the  spring 
time,  Leo  and  Virgo  may  be  seen  rising  above  the 
eastern  horizon  in  the  early  evening  hours.  When  the 
first  snow  flies  in  the  late  fall  and  the  birds  have  all 
gone  southward  the  belt  of  Orion  appears  in  the  east 
and  Cygnus  dips  low  in  the  west.  When  we  once 
come  to  know  brilliant  blue-white  Vega,  ruddy  Arc- 
turus,  golden  Capella  and  sparkling  Sirius  we  watch 
for  them  to  return  each  in  its  proper  season  and  greet 
them  as  old  friends. 

In  the  following  pages  we  give  for  each  month  the 
constellations  or  star-groups  that  are  nearest  to  our 
meridian,  that  is,  that  lie  either  due  north  or  due  south 
or  exactly  overhead  in  the  early  part  of  the  month 
and  the  early  part  of  the  evening. 

We  do  not  need  to  start  our  study  of  the  constella- 
tions in  January.  We  may  start  at  any  month  in  the 
year  and  we  will  find  the  constellations  given  for  that 
month  on  or  near  the  meridian  at  the  time  indicated. 

In  using  the  charts  or  diagrams  of  the  constellations, 
we  should  hold  them  in  an  inverted  position  with  the 
top  of  the  page  toward  the  north  or  else  remember 
that  the  left-hand  side  of  the  page  is  toward  the  east 
and  the  right-hand  side  of  the  page  toward  the  west, 
which  is  the  opposite  of  the  arrangement  for  charts 
and  maps  of  the  earth's  surface. 

We  should  also  bear  in  mind  that  the  constellations 
are  all  continually  shifting  westward  for  the  stars 
and  the  moon  and  the  planets  as  well  as  the  sun  rise 


THE  CONSTELLATIONS  5 

daily  in  the  east  and  set  in  the  west.  This  is  due 
to  the  fact  that  the  earth  is  turning  in  the  opposite 
direction  on  its  axis,  that  is  from  west  to  east.  In 
twenty-four  hours  the  earth  turns  completely  around 
with  respect  to  the  heavens  or  through  an  angle  of 
360°,  so  in  one  hour  it  turns  through  an  angle  of 
360°  -f-  24  or  15°.  As  a  result  the  stars  appear  to 
shift  westward  15°  every  hour.  This  is  a  distance 
about  equal  in  length  to  the  handle  of  the  Big  Dipper, 
which  I  am  sure  we  all  know,  even  if  we  do  not 
know  another  constellation  in  the  heavens. 

If,  then,  we  look  at  the  heavens  at  a  later  hour 
than  that  for  which  the  constellations  are  given  we 
will  find  them  farther  westward  and  if  our  time  of 
observation  is  earlier  in  the  evening  than  the  hour 
mentioned  we  will  find  them  farther  eastward. 

In  the  course  of  a  year  the  earth  makes  one  trip 
around  the  sun  and  faces  in  turn  all  parts  of  the 
heavens.  That  is,  it  turns  through  an  angle  of  360° 
with  respect  to  the  heavens  in  a  year  or  through  an 
angle  of  360°  -f-  12  or  30°  in  one  month.  So  as  a 
result  of  our  revolution  around  the  sun,  which  is  also  in 
a  west  to  east  direction,  we  see  that  all  the  constella- 
tions are  gradually  shifting  westward  at  the  rate  of 
30°  a  month.  It  is  for  this  reason  that  we  see  different 
constellations  in  different  months,  and  it  is  because 
of  the  turning  of  the  earth  on  its  axis  that  we  see 
different  constellations  at  different  hours  of  the  night 

If  we  should  sit  up  from  sunset  to  sunrise  and  watch 
the  stars  rise  in  the  east,  pass  the  meridian  and  set 


6          ASTRONOMY  FOR  YOUNG  FOLKS 

in  the  west — as  the  sun  does  by  day — we  should  see 
in  turn  the  same  constellations  that  are  to  pass  across 
the  heavens  in  the  next  six  months.  This  is  because  in 
twelve  hours'  time  we  are  carried  through  the  same 
angle  with  respect  to  the  heavens  by  the  earth's  rota- 
tion on  its  axis  that  we  are  in  the  next  six  months  by 
the  motion  of  the  earth  around  the  sun. 

Let  us  suppose  then  that  the  time  we  choose  for  our 
observation  of  the  heavens  is  the  last  of  the  month 
while  our  charts  are  given  for  the  first  of  the  month. 
We  must  look  then  farther  westward  for  our  con- 
stellations just  as  we  must  look  farther  westward  if 
we  chose  a  later  hour  in  the  evening  for  our  observa- 
tions. Let  us  suppose  that  we  choose  for  our  time 
of  observation  half-past  eight  in  the  early  part  of 
December.  On  or  close  to  the  meridian  we  will  find 
the  constellations  outlined  in  the  charts  for  Decem- 
ber. To  the  east  of  the  meridian  we  will  find  the 
constellations  that  are  given  for  January  and  Feb- 
ruary, and  to  the  west  of  the  meridian  the  constella- 
tions that  are  given  for  November  and  October. 
So  if  we  are  particularly  ambitious  or  wish  to  be- 
come acquainted  with  the  constellations  more  rapidly 
we  may  study  at  the  same  time  the  constellations  for 
the  preceding  months  now  west  of  the  meridian  and 
the  constellations  for  the  following  months  now  east 
of  the  meridian  as  well  as  the  constellations  for  the 
month  which  will  be  due  north  or  south  or  directly 
overhead  as  the  case  may  be. 

If  we  were  able  to  see  the  stars  by  day  as  well  as 


THE  CONSTELLATIONS  7 

by  night  we  would  observe  that  as  the  days  go  by 
the  sun  is  apparently  moving  continuously  eastward 
among  certain  constellations.  This  is  a  result  of  the 
earth's  actual  motion  around  the  sun  in  the  same 
direction. 

The  apparent  path  of  the  sun  among  the  stars  is 
called  the  ecliptic  and  the  belt  of  the  heavens  eight 
degrees  wide  on  either  side  of  the  ecliptic  is  called  the 
zodiac.  The  constellations  that  lie  within  this  belt 
of  the  zodiac  are  called  zodiacal  constellations.  The 
zodiac  was  divided  by  the  astronomer  Hipparchus,  who 
lived  161-126  B.C.,  into  twelve  signs  30°  wide, 
and  the  signs  were  named  for  the  constellations  lying 
at  that  time  within  each  (of  these  divisions.  These 
zodiacal  constellations  are  Aries,  Taurus,  Gemini,  Can- 
cer, Leo,  Virgo,  Libra,  Scorpio,  Sagittarius,  Capri- 
cornus,  Aquarius  and  Pisces.  With  the  exception  of 
Libra,  the  Scales,  all  of  these  constellations  are  named 
for  people  or  animals  and  the  word  zodiac  is  derived 
from  the  Greek  word  meaning  'W  animals." 

Each  month  the  sun  moves  eastward  30°  through  one 
of  these  zodiacal  constellations.  In  the  days  of  Hip- 
parchus the  sun  was  in  Aries  at  the  beginning  of 
spring,  at  the  point  where  the  ecliptic  crosses  the  celes- 
tial equator — which  lies  directly  above  the  earth's  equator. 
This  point  where  the  ecliptic  crosses  the  equator  was 
then  known  as  the  First  Point  in  Aries.  The  autumnal 
equinox  was  180°  distant  in  Aquarius  and  the  two 
points  were  called  the  equinoxes  because  when  the  sun 
is  at  either  equinox  the  day  and  night  are  equal  in 


8          ASTRONOMY  FOR  YOUNG  FOLKS 

length  all  over  the  world.  Now  for  certain  reasons 
which  we  will  not  explain  here  the  equinoctial  points 
are  not  fixed  in  position  but  shift  gradually  westward 
at  the  rate  of  1°  in  70  years.  It  is  as  if  the  equinoxes 
were  advancing  each  year  to  meet  the  sun  on  its 
return  and  their  westward  motion  is  therefore  called 
"The  Precession  of  the  Equinoxes." 

Since  the  days  of  Hipparchus  this  motion  has 
amounted  to  about  30°  so  that  the  constellations  no 
longer  occupy  the  signs  of  the  zodiac  that  bear  their 
names. 

The  sun  is  now  in  Pisces  instead  of  Aries  at  the 
beginning  of  spring  and  in  Virgo  instead  of  Aquarius 
at  the  beginning  of  fall. 

Not  only  the  sun  but  the  moon  and  planets  as 
well  move  through  the  zodiacal  constellations.  In 
fact  a  limit  for  the  zodiac  of  8°  on  either  side  of 
the  ecliptic  was  chosen  because  it  marks  the  extent  of 
the  excursions  of  the  moon  and  planets  from  the 
ecliptic.  Neither  moon  nor  planets  will  be  found  at 
a  greater  distance  than  8°  on  either  side  of  the 
ecliptic. 

For  convenience  in  determining  the  positions  of  the 
heavenly  bodies  the  astronomer  assumes  that  they  lie 
upon  the  surface  of  a  celestial  sphere  that  has  its 
center  at  the  center  of  the  earth. 

The  north  pole  of  the  celestial  sphere  lies  directly 
above  the  north  pole  of  the  earth  and  the  south  pole 
of  the  celestial  sphere  directly  above  the  south  pole 


THE  CONSTELLATIONS  9 

of  the  earth.  The  celestial  equator  is  the  great  circle 
of  the  celestial  sphere  that  lies  midway  between  its 
north  and  south  poles  and  directly  above  the  earth's 
equator.  The  ecliptic  is  also  a  great  circle  of  the 
celestial  sphere  and  cuts  the  celestial  equator  at  an 
angle  of  23^°  in  the  two  points  180°  apart  known 
as  the  equinoctial  points,  of  which  we  have  already 
spoken. 

The  zodiacal  constellations  lie  nearly  overhead  within 
the  tropics  and  can  be  seen  to  advantage  all  over  the 
world  except  in  polar  regions. 

For  every  position  of  the  earth's  surface  except  at 
the  equator  we  have  also  our  circumpolar  constellations 
which  are  the  ones  that  never  pass  below  the  horizon 
for  the  place  of  observation. 

In  40°  N.  Latitude  the  Big  Dipper  is  a  circumpolar 
constellation  for  it  is  above  the  horizon  at  all  hours  of 
the  day  and  night  and  all  times  of  the  year.  If  our 
latitude  is  40°  N.,  all  stars  within  40°  of  the  north 
pole  of  the  heavens  are.  circumpolar  and  never  set, 
while  stars  within  40°  of  the  south  pole  of  the  heavens 
never  rise.  All  other  stars  rise  and  set  daily. 

If  we  were  at  the  north  pole  all  stars  within  90° 
of  the  north  pole  of  the  heavens  would  be  circumpolar 
and  would  describe  daily  circuits  of  the  pole  parallel 
to  the  horizon  remaining  always  above  it. 

If  we  were  at  the  equator  all  stars  within  zero 
degrees  of  either  pole  would  be  circumpolar,  that  is  no 


10        ASTRONOMY  FOR  YOUNG  FOLKS 

stars  would  be  circumpolar,  all  stars  rising  and  setting 
daily. 

As  a  general  rule,  then,  we  may  say  that  stars  within 
an  angular  distance  of  the  nearest  pole  of  the  heavens 
equal  to  the  latitude  never  set  and  stars  within  an  equal 
distance  of  the  opposite  pole  never  rise  while  all  stars 
outside  of  these  limits  rise  and  set  daily. 

The  beginner  who  attempts  to  make  the  acquain- 
tance of  the  principal  stars  and  constellations  occasion- 
ally may  find  a  bright  star  in  a  constellation  that  is 
not  noted  in  the  diagrams.  In  this  case  he  has  prob- 
ably happened  upon  one  of  the  bright  planets. 

It  is  not  possible  to  include  the  planets  in  our  dia- 
grams for  the  reason  that  they  are  not  fixed  in  position 
but  apparently  wander  among  the  stars.  The  name 
planet  is,  in  fact,  derived  from  a  Greek  word  meaning 
"wanderer."  The  stars  shine  by  their  own  light  but 
the  planets  shine  only  by  reflected  light  from  the  sun. 
Of  the  seven  planets,  in  the  solar  system  additional  to 
our  own  planet  earth,  there  are  two,  Uranus  and 
Neptune  that  we  need  not  consider  for  Neptune  is  not 
visible  without  the  aid  of  a  telescope  and  Uranus  is 
fainter  than  any  of  the  stars  included  in  our  diagrams. 

Mercury  will  never  appear  except  in  the  morning  or 
evening  twilight,  when  none  but  the  very  brightest 
stars  are  visible,  since  it  never  departs  far  from  the 
sun.  It  will  only  be  seen  under  certain  favorable  con- 
ditions, and  usually  it  will  escape  our  notice  altogether 


THE  CONSTELLATIONS  11 

unless  we  know  exactly  where  to  look  for  it  although 
there  are  but  two  or  three  stars  in  the  heavens  that 
surpass  it  in  brightness. 

Venus,  we  will  probably  never  mistake  for  any  star 
in  the  heavens  for  it  far  surpasses  all  stars  in  bright- 
ness. It  will  always  be  seen  in  the  west  after  sunset 
or  in  the  east  before  sunrise  and  it  is  never  seen  more 
than  three  hours  before  or  after  the  sun. 

This  leaves  us  but  three  planets,  Jupiter,  Saturn  and 
Mars  that  we  may  mistake  for  bright  stars.  There  is 
little  chance  that  Jupiter  will  be  thus  mistaken  for 
it  also  is  far  brighter  than  all  of  the  stars  except  Sirius 
which  differs  greatly  from  Jupiter  in  color.  Sirius  is 
a  brilliant  white  and  Jupiter  is  a  golden  yellow.  The 
planets  do  not  twinkle  as  the  stars  do  and  this  is 
particularly  true  of  Jupiter  which  is  remarkable  for  the 
quiet  steadiness  of  its  yellow  light.  This  alone  would 
serve  to  identify  it. 

Saturn  is  probably  mistaken  for  a  star  oftener  than 
any  of  the  other  planets.  It  moves  so  slowly  among 
the  stars  that  we  would  have  to  watch  it  for  a  number 
of  successive  evenings  before  we  could  discover  that 
it  is  moving  with  respect  to  the  stars.  Saturn  is  yel- 
lowish in  color  and  we  can  probably  best  distinguish  it 
by  the  steadiness  of  its  light.  If  we  find  in  one  of  the 
zodiacal  groups  of  stars — for  the  planets  appear  among 
no  other  constellations — a  bright  yellowish  star  where 
no  bright  star  is  indicated  on  the  diagram  we  may  be 


12        ASTRONOMY  FOR  YOUNG  FOLKS 

reasonably  certain  that  we  have  found  the  planet  Saturn. 

Mars  is  the  only  planet  that  is  reddish  in  color. 
Once  in  fifteen  or  seventeen  years,  when  it  is  particularly 
near  to  the  earth,  it  surpasses  even  Jupiter  in  brightness, 
but  ordinarily  it  appears  no  more  brilliant  than  one 
of  the  brighter  stars.  There  are  only  two  stars  with 
which  we  are  likely  to  confuse  Mars, — Aidebaran  and 
Antares — which  are  very  similar  to  it  in  color,  and,  at 
times,  in  brightness.  Moreover,  both  of  these  stars 
are  zodiacal  stars  and  Mars  frequently  passes  through 
the  constellations  to  which  they  belong.  There  should 
be  no  trouble  about  identifying  Aldebaran  and  Antares, 
however,  from  their  distinctive  positions  in  the  dia- 
grams so  that  any  other  reddish  star  appearing  in  any 
of  the  zodiacal  groups  we  may  feel  certain  is  the 
planet  Mars. 

In  the  following  diagrams  of  the  constellations  the 
brightest  and  most  conspicuous  stars,  called  first-mag- 
nitude stars,  are  represented  by  white  stars.  These 
are  the  stars  we  should  all  be  able  to  recognize  and  call 
by  name  and  in  every  instance  the  name  of  a  first-mag- 
nitude star  is  given  on  the  diagram.  All  other  stars 
are  represented  by  circles,  and  the  size  of  the  circle  is 
an  indication  of  the  brightness  of  the  star. 

Stars  visible  without  the  aid  of  a  telescope  are 
referred  to  usually  as  "naked-eye  stars."  They  are 
classed  as  first,  second,  third,  four,  fifth  or  sixth 
magnitude  stars,  according  to  their  relative  brightness. 


THE  CONSTELLATIONS  13 

A  star  of  the  first  magnitude  is  about  two  and  one-half 
times  brighter  than  a  star  of  the  second  magnitude, 
which  in  turn  is  two  and  one-half  times  brighter  than 
a  star  of  the  third  magnitude  and  so  on.  A  first- 
magnitude  star  is,  then,  one  hundred  times  brighter  than 
a  sixth  magnitude  star  which  is  the  faintest  star  that 
can  be  seen  without  the  aid  of  the  telescope. 

This  ratio  between  successive  magnitudes  continues 
among  the  telescopic  stars.  A  star  of  the  sixth 
magnitude  is  one  hundred  times  brighter  than  a  star 
of  the  eleventh  magnitude  which  in  turn  is  one  hundred 
times  brighter  than  a  star  of  the  sixteenth  magnitude. 

The  faintest  stars  that  can  be  seen  visually  in  the 
greatest  telescopes  are  of  the  seventeenth  or  eighteenth 
magnitude,  though  stars  two  or  three  magnitudes 
fainter  can  be  photographed. 

The  faintest  stars  shown  in  the  diagrams  are  fifth- 
magnitude  stars  and  stars  of  this  magnitude  as  well 
as  stars  of  the  fourth  magnitude  are  only  given  when 
needed  to  fill  out  the  distinctive  outlines  of  the  con- 
stellations which  have  been  formed  by  connecting  the 
principal  stars  in  each  group  by  dotted  lines. 

All  stars  of  first,  second  and  third  magnitude  are 
given  in  the  diagrams  without  exceptions  as  such  stars 
are  visible  to  everyone  on  clear  nights. 

The  constellations  given  in  the  following  pages  in- 
clude practically  all  of  the  constellations  that  can  be 
seen  in  40°  N.  Latitude.  A  diagram  is  given  for  each 
constellation. 


14        ASTRONOMY  FOR  YOUNG  FOLKS 

In  this  latitude  it  is  impossible  to  see  the  constella- 
tions of  the  southern  hemisphere  that  lie  within  40° 
of  the  south  pole  of  the  heavens.  A  brief  chapter 
with  diagram  treats  of  these  constellations  that  are 
invisible  in  mid-latitudes  of  the  northern  hemisphere. 


II 

JANUARY 

ONE  of  the  most  easily  recognized  constellations  in 
the  heavens  is  Taurus,  The  Bull,  a  zodiacal  group 
which  lies  just  south  of  the  zenith  in  our  latitudes  in 
the  early  evening  hours  about  the  first  of  January. 

Taurus  is  distinguished  by  the  V-shaped  group  of 
The  Hyades,  which  contains  the  bright,  red,  first-mag- 
nitude star  Aldebaran,  representing  the  fiery  eye  of 
the  bull.  It  also  contains  the  famous  cluster  of  faint 
stars  known  as  The  Pleiades,  lying  a  short  distance 
northwest  of  The  Hyades. 

No  group  of  stars  is  more  universally  known  than 
The  Pleiades.  All  tribes  and  nations  of  the  world, 
from  the  remotest  days  of  recorded  history  up  to  the 
present  time,  have  sung  the  praises  of  The  Pleiades. 
They  were  "The  Many  Little  Ones"  of  the  Babylonians, 
"The  Seven  Sisters"  of  the  Greeks,  "The  Seven  Broth- 
ers" of  the  American  Indians,  "The  Hen  and  Chick- 
ens" of  many  nations  of  Europe,  "The  Little  Eyes"  of  the 
South  Sea  Islanders.  They  were  honored  in  the  re- 
ligious ceremonies  of  the  Aztecs,  and  the  savage  tribes 
of  Australia  danced  in  their  honor.  Many  early  tribes 
of  men  began  their  year  with  November,  the  Pleiad 

15 


16        ASTRONOMY  FOR  YOUNG  FOLKS 

month;  and  on  November  17th,  when  The  Pleiades 
crossed  the  meridian  at  midnight,  it  was  said  that  no 
petition  was  ever  presented  in  vain  to  the  kings  of 
ancient  Persia. 


JANUARY — TAURUS 

Poets  of  all  ages  have  felt  the  charm  of  The  Pleiades. 
Tennyson  gives  the  following  beautiful  description  of 
The  Pleiades  in  Locksley  Hall: 
"Many  a  night  I  saw  the  Pleiades,  rising  through  the 

mellow  shade, 
Glitter   like   a    swarm   of   fireflies   tangled   in   a   silver 

braid/' 


JANUARY  17 

A  well-known  astronomer,  not  so  many  years  ago, 
also  felt  the  mysterious  charm  of  The  Pleiades  and 
seriously  expressed  the  belief  that  Alcyone,  the  bright- 
est star  of  The  Pleiades,  was  a  central  sun  about  which 
all  other  suns  were  moving.  But  we  know  that  there 
is  no  foundation  whatever  for  such  a  belief. 

A  fairly  good  eye,  when  the  night  is  clear  and  dark, 
will  make  out  six  stars  in  this  group  arranged  in  the 
form  of  a  small  dipper.  A  seventh  star  lies  close  to 
the  star  at  the  end  of  the  handle  and  is  more  difficult 
to  find.  It  is  called  Pleione,  and  is  referred  to  in 
many  legends  as  the  lost  Pleiad.  Persons  gifted  with 
exceptionally  fine  eyesight  have  made  out  as  many  as 
eleven  stars  in  the  group;  and  with  the  aid  of  an  or- 
dinary opera-glass,  anyone  can  see  fully  one  hundred 
stars  in  this  cluster.  Astronomers  have  found  that  The 
Pleiades  cluster  contains  at  least  two  hundred  and  fifty 
stars,  all  drifting  slowly  in  the  same  general  direction 
through  space,  and  that  the  entire  group  is  enveloped  in 
a  fiery,  nebulous  mist  which  is  most  dense  around  the 
brightest  stars.  It  is  not  known  whether  the  stars  are 
being  formed  from  the  nebula  or  whether  the  nebula  is 
being  puffed  off  from  the  stars.  The  brightest  star, 
Alcyone,  is  at  least  two  hundred  times  more  brilliant 
than  our  own  sun,  and  all  of  the  brighter  stars  in  the 
group  surpass  the  sun  many  times  in  brightness.  It 
is  believed  that  this  cluster  is  so  large  that  light  takes 
many  years  to  cross  from  one  end  of  it  to  the  other, 
and  that  it  is  so  far  from  the  earth  that  its  light  takes 


18        ASTRONOMY  FOR  YOUNG  FOLKS 

over  three  centuries  to  reach  us,  traveling  at  the  rate 
of  186,000  miles  a  second. 

The  Hyades  is  a  group  of  stars  scarcely  less  famous 
than  The  Pleiades,  and  the  stars  in  the  group  also  form 
a  moving  cluster  of  enormous  extent  at  a  distance  of 
140  light-years  from  the  earth. 

Among  the  ancients,  The  Hyades  were  called  the  rain- 
stars,  and  the  word  Hyades  is  supposed  to  come  from 
the  Greek  word  for  rain.  Among  the  many  superstitions 
of  the  past  was  the  belief  that  the  rising  or  setting  of 
a  group  of  s.tars  with  the  sun  had  some  special  influence 
over  human  affairs.  Since  The  Hyades  set  just  after 
the  sun  in  the  showery  springtime  and  just  before  sun- 
rise in  the  stormy  days  of  late  fall,  they  were  always 
associated  with  rain.  In  Tennyson's  Ulysses  we  read: 

"Through  '  scudding   drifts   the    rainy   Hyades 
Vex'd  the  dim  sea." 

Trie  Hyades  outline  the  forehead  of  Taurus,  while 
two  bright  stars  some  distance  to  the  northeast  of  the 
V  form  the  tips  of  the  horns.  Only  the  head  and  fore- 
quarters  of  the  bull  are  shown  in  the  star-atlases  that 
give  the  mythological  groups,  for,  according  to  one 
legend,  he  is  swimming  through  the  sea  and  the  rest 
of  his  body  is  submerged.  According  to  another  legend, 
Taurus  is  charging  down  upon  Orion,  The  Warrior, 
represented  by  the  magnificent  constellation  just  to 
the  southeast  of  Taurus,  of  which  we  shall  have  more 
to  say  next  month. 

Aldebaran  is  the  Arabic  word  for  "The  Hindmost," 


JANUARY  19 

and  the  star  is  so  called  because  it  follows  The  Pleiades 
across  the  sky.  It  is  one  of  the  most  beautiful  of  all 
the  many  brilliant  stars  visible  at  this  time  and  we 
might  profit  by  following  the  advice  of  Mrs.  Sigourney 
in  The  Stars: 

"Go  forth  at  night 

And  talk  with  Aldebaran,  where  he  flames 
In  the  cold   forehead  of  the  wintry  sky." 

Next  to  Aldebaran  in  the  V  is  the  interesting  double 
star  Theta,  which  we  can  see  as  two  distinct  stars 
without  a  telescope. 

Directly  south  of  Taurus  is  Eridanus,  sometimes 
called  Fluvius  Eridanus,  or  The  River  Eridanus.  Start- 
ing a  little  to  the  southeast  of  Taurus,  close  to  the 
brilliant  blue-white  star  Rigel  in  Orion,  it  runs  to  the 
westward  for  a  considerable  distance  in  a  long  curving 
line  of  rather  faint  stars,  bends  sharply  southward  for 
a  short  distance,  then  curves  backward  toward  the  east 
once  more,  and,  after  running  for  some  distance,  makes 
another  sharp  curve  to  the  southwest  and  disappears 
below  the  southern  horizon.  Its  course  is  continued  far 
into  the  southern  hemisphere.  Its  brightest  star, 
Achernar,  is  a  star  of  the  first  magnitude,  but  it  lies 
below  the  horizon  in  our  latitudes. 

Eridanus  contains  no  star  of  particular  interest  to 
us.  Most  of  the  numerous  stars  that  mark  its  course 
are  of  the  fourth  and  fifth  magnitude.  It  contains  but 
two  stars  of  the  third  magnitude,  one  at  the  beginning 
of  its  course  and  one  close  to  the  southwestern  horizon. 


20 


ASTRONOMY  FOR  YOUNG  FOLKS 


The  beautiful  constellation  of  Perseus  lies  just  to  the 
north  of  Taurus  and  should  rightfully  be  considered 
among  the  constellations  lying  nearest  to  the  meridian  in 
January,  but  we  give  this  constellation  among  the  star 


JANUARY — ERIDANUS 

groups  for  December  because  of  its  close  association 
with  the  nearby  constellations  Andromeda  and  Pegasus 
in  legend  and  story. 


m 

FEBRUARY 

ACROSS  the  meridian,  due  south,  between  eight  and 
nine  o'clock  in  the  evening  in  the  early  part  of  Feb- 
ruary, lies  Orion,  The  Warrior,  generally  considered 
to  be  the  finest  constellation  in  the  heavens.  Orion 
is  directly  overhead  at  the  equator,  and  so  is  seen  to 
advantage  from  all  parts  of  the  world  except  the 
extreme  northern  and  southern  polar  regions. 

A  group  of  three  faint  stars  outlines  the  head  of 
Orion.  His  right  shoulder  is  marked  by  the  deep-red, 
first-magnitude  star  Betelgeuze  (meaning  armpit),  and 
his  left  shoulder  by  the  bright  white  star  Bellatrix, 
The  Amazon.  Orion  stands  facing  Taurus,  The  Bull, 
and  brandishes  in  his  right  hand  a  club,  outlined  by  a 
number  of  faint  stars  extending  from  Betelgeuze  toward 
the  northeast.  The  top  of  the  club  lies  near  the  tips 
of  the  horns  of  Taurus.  In  his  left  hands  he  holds  up  a 
lion's  skin,  which  we  can  trace  in  another  curving 
line  of  faint  stars  to  the  west  and  northwest  of  Bella- 
trix. The  brilliant,  blue-white,  first-magnitude  star 
Rigel  lies  in  the  left  foot,  and  the  second-magnitude 
star  Saiph,  a  little  to  the  east  of  Rigel,  is  in  the  right 
knee.  Three  evenly  spaced  stars  lying  in  a  straight 
line  that  is  exactly  three  degrees  in  length  form  the 

21 


22        ASTRONOMY  FOR  YOUNG  FOLKS 

Belt  of  Orion,  and  from  the  Belt  hangs  the  Sword  of 
Orion,  outlined  by  three  faint  stars.  The  central  star 
in  the  Sword  appears  somewhat  blurred  and  is  the 
multiple  star  Theta,  in  the  midst  of  the  great  Orion 
nebula,  the  finest  object  of  its  kind  in  the  heavens. 
Entangled  in  the  meshes  of  this  glowing  nebula  are  a 
number  of  brilliant  suns,  appearing  to  us  as  faint  stars 
because  of  their  great  distance.  The  star  Theta,  in 
the  heart  of  the  nebula,  is  seen  with  a  powerful  telescope 
to  consist  of  six  stars;  that  is,  it  is  a  sextuple  star. 
Even  with  a  small  telescope,  four  of  these  stars  can 
readily  be  seen,  arranged  in  the  form  of  a  small  tra- 
pezium. The  lowest  star  in  the  Sword  is  a  triple 
star,  and  the  entire  constellation  abounds  in  double, 
triple,  and  multiple  stars. 

From  the  central  portion  of  the  nebula  extend  many 
branches  and  streamers  of  nebulous  light,  and  it  is 
known  that  the  entire  constellation  of  Orion  is  en- 
wrapped in  the  folds  of  this  nebulosity,  which  forms 
a  glowing,  whirling  mass  of  fiery  gases  in  rapid  rota- 
tion. This  constellation  is  remarkable  for  the  fact  that 
all  of  its  brighter  stars,  with  the  exception  of  the  deep- 
red  Betelgeuze,  form  one  enormous,  connected  group 
of  stars.  They  are  all  more  or  less  associated  with 
the  great  nebula  and  its  branches,  and  are  all  extremely 
hot,  white  or  bluish-white  stars,  known  as  helium 
stars,  because  the  gas  helium  is  so  conspicuous  in  their 
atmospheres.  The  Orion  stars  are  the  hottest  and 
brightest  of  all  the  stars. 

Blazing  Rigel,   Bellatrix,  and   Saiph,   marking  three 


FEBRUARY 


23 


corners  of  the  great  quadrilateral,  of  which  Betelgeuze 
marks  the  fourth  corner,  are  all  brilliant  helium  stars. 
So  are  the  three  stars  in  the  Belt  -and  the  fainter  stars 
in  the  Sword  and  the  great  nebula. 


FEBRUARY — ORION 

It  has  been  estimated  that  the  great  Orion  group 
of  stars  is  over  six  hundred  light-years  from  the  earth, 
or  about  forty  million  times  more  distant  than  the 
sun.  For  more  than  six  centuries  the  rays  of  light 
that  now  enter  our  eyes  from  these  stars  have  been 
traveling  through  space  with  the  speed  of  lightning. 
So  we  see  Orion  not  as  it  exists  today,  but  as  it  was 


24    ASTRONOMY  FOR  YOUNG  FOLKS 

six  centuries  ago.  The  extent  of  the  Orion  group 
of  stars  is  also  inconceivably  great.  Even  the  central 
part  of  the  great  nebula,  which  appears  to  our  unaided 
eyes  only  as  a  somewhat  fuzzy  star,  would  extend 
from  here  to  the  nearest  star  and  beyond,  while  our 
entire  solar  system  would  be  the  merest  speck  in  its 
midst. 

Betelgeuze,  the  red  star  that  marks  the  right  shoulder 
of  Orion,  is,  as  we  have  said,  not  a  member  of  the 
Orion  group.  It  has  been  estimated  that  it  is  about 
two  hundred  light-years  from  the  earth,  or  only 
about  one-third  as  far  away  as  the  other  stars  of  the 
constellation. 

Betelgeuze  very  recently  has  attracted  universal  at- 
tention, and  will  probably  be  considered  an  object  of 
historic  interest  in  the  future,  because  it  is  the  first 
star  to  have  its  diameter  measured  with  the  new 
Michelson  interferometer,  which  is  now  being  used  so 
successfully  to  measure  the  diameters  of  the  largest 
stars.  The  truly  sensational  discovery  has  been  made 
that  Betelgeuze  is  a  supergiant  of  the  universe,  with  a 
diameter  of  about  275,000,000  miles.  Our  own  sun, 
which  is  known  as  a  "dwarf"  star,  has  a  diameter  of 
864,000  miles.  That  is,  Betelgeuze  would  make  about 
thirty  million  suns  the  size  of  our  own.  If  placed  at 
the  center  of  the  solar  system,  it  would  fill  all  of  the 
space  within  the  orbit  of  Mars;  and  the  planets 
Mercury,  Venus,  and  the  Earth  would  lie  far  beneath 
its  surface.  Measurements  of  the  diameters  of  other 
giant  stars  which  are  now  being  made  with  the  in- 


FEBRUARY  25 

terferometer  give  results  quite  as  startling  as  have  been 
obtained  in  the  case  of  Betelgeuze;  and  it  has  been 
found  that  several  of  these  stars  may  even  exceed  Betel- 
geuze in  size.  Such  a  star  is  Antares,  the  fiery-red 
star  in  the  heart  of  Scorpio,  which  is  such  a  conspicu- 
ous object  in  the  summer  evening  skies.  All  these 
huge  stars  are  deep  red  in  color,  and  some  of  them  vary 
irregularly  in  brightness.  Betelgeuze  is  one  of  the 
stars  that  changes  in  brightness  in  a  peculiar  manner 
from  time  to  time.  When  shining  with  its  greatest 
brilliancy  it  is  a  brighter  object  than  the  nearby 
Aldebaran,  in  Taurus ;  but  a  few  months  or  a  year  later 
it  may  lose  so  much  of  its  light  as  to  be  decidedly 
inferior  to  Aldebaran.  We  may  note  for  ourselves 
this  remarkable  change  in  the  brightness  of  Betelgeuze 
by  comparing  the  two  stars  from  time  to  time. 

Directly  south  of  Orion  lies  the  small  constellation 
of  Lepus,  The  Hare,  which  is  made  up  of  third- 
magnitude  and  fourth-magnitude  stars.  The  four 
brighter  stars  -are  arranged  in  the  form  of  a  small,  but 
distinct,  quadrilateral,  or  four-sided  figure,  which  may 
be  easily  seen  in  our  latitudes.  The  small  constellation 
of  Columba,  The  Dove,  which  lies  just  south  of  Lepus, 
is  so  close  to  the  horizon  that  it  can  not  be  seen  to 
advantage  in  the  mid-latitudes  of  the  northern  hemi- 
sphere. Neither  Lepus  nor  Columba  contain  any  object 
of  unusual  interest. 

Due  north  of  Orion,  and  lying  in  the  zenith  at  this 
time,  is  Auriga,  The  Charioteer,  represented,  strange 
to  say,  with  Capella,  a  goat,  in  his  arms.  The  beautiful 


26        ASTRONOMY  FOR  YOUNG  FOLKS 

first-magnitude  star  Capella,  golden-yellow  in  color, 
serves  us  in  identifying  the  constellation.  Close  at  hand 
are  The  Kids,  represented  by  a  group  of  three  faint 
stars.  Capella  is  one  of  the  most  brilliant  stars  of  the 


FEBRUARY — AURIGA 

northern  hemisphere.  It  is  almost  exactly  equal  in 
brightness  to  Arcturus  and  Vega,  stars  conspicuous  in 
the  summer  months,  and  it  is  a  shade  brighter  than 
magnificent  blue-white  Rigel  in  Orion.  Capella  is 
about  fifty  light-years  distant  from  the  earth  and  is 
fully  two  hundred  times  more  brilliant  than  our  own 
sun.  At  the  distance  of  Capella,  the  sun  would  appear 


FEBRUARY  27 

to  be  considerably  fainter  than  any  one  of  the  three 
stars  in  the  nearby  group  of  The  Kids. 

Capella  is  attended  by  a  companion  star  so  close  to  its 
brilliant  ruler  that  it  can  not  be  seen  as  a  separate  star 
save  with  the  aid  of  the  most  powerful  telescopes.  Its 
distance  from  Capella  has  been  very  accurately  mea- 
sured, however,  by  means  of  the  interferometer,  which 
is  giving  us  the  measurements  of  the  diameters  of  the 
giant  stars.  It  is  known  that  this  companion  sun  is 
closer  to  Capella  than  our  planet  earth  is  to  the  sun. 

At  no  time  of  the  year  shall  we  find  near  the  meridian 
so  many  brilliant  and  beautiful  stars  as  appear  in  the 
month  of  February  at  this  time  in  the  evening.  In 
addition  to  Capella,  which  is  one  of  the  three  most 
brilliant  stars  in  the  northern  hemisphere  of  the 
heavens,  we  have,  in  Orion  alone,  two  stars  of  the 
first  magnitude,  Betelgeuze  and  Rigel,  and  five  stars 
of  the  second  magnitude,  Bellatrix  and  Saiph  and  the 
three  stars  in  the  Belt.  In  addition,  we  have  not  far 
distant  in  the  western  sky,  fiery  Aldebaran  in  Taurus, 
and  close  on  the  heel  of  Orion  in  the  east,  Sirius,  the 
brightest  star  in  the  heavens,  in  the  constellation  of 
Canis  Major,  The  Greater  Dog,  as  well  as  the  first- 
magnitude  star  Procyon  in  Canis  Minor,  The  Lesser 
Dog.  Of  these  two  groups  we  shall  have  more  to  say 
under  the  constellations  for  March. 


IV 
MARCH 

To  the  southeast  of  Orion  and  almost  due  south  at 
eight  o'clock  in  the  evening  on  the  first  of  March  lies 
the  constellation  of  Canis  Major,  The  Greater  Dog, 
containing  Sirius,  the  Dog-star,  which  far  surpasses 
all  other  stars  in  the  heavens  in  brilliancy. 

Sirius  lies  almost  in  line  with  the  three  stars  that 
form  the  Belt  of  Orion.  We  shall  not  have  the  slightest 
difficulty  in  recognizing  it,  owing  to  its  surpassing 
brilliancy  as  well  as  to  the  fact  that  it  follows  so 
closely  upon  the  heels  of  Orion. 

Sirius  is  the  Greek  for  "scorching"  or  "sparkling," 
and  the  ancients  attributed  the  scorching  heat  of 
summer  to  the  fact  that  Sirius  then  rose  with  the 
sun.  The  torrid  days  of  midsummer  they  called  the 
"dog-days"  for  this  reason,  and  we  have  retained  the 
expression  to  the  present  time.  Since  Sirius  was 
always  associated  with  the  discomforts  of  the  torrid 
season,  it  did  not  have  an  enviable  reputation  among 
the  Greeks.  We  find  in  Pope's  translation  of  the  Iliad 
this  reference  to  Sirius : 

'Terrific  glory!   for  his  burning  breath 
Taints  the  red  air  with  fever,  plagues,  and  death." 

28 


MARCH 


29 


In  Egypt,  however,  many  temples  were  dedicated  to 
the  worship  of  Sirius,  for  the  reason  that  some  five 
thousand  years  ago  it  rose  with  the  sun  at  the  time  of 
the  summer  solstice,  which  marks  the  beginning  of 
summer,  and  heralded  the  approaching  inundation  of 
the  Nile,  which  was  an  occasion  for  great  rejoicing 
among  the  Egyptians.  It  was,  therefore,  called  the 
Nile  Star  and  regarded  by  them  with  the  greatest 
reverence. 

Sirius  is  an  intensely  white  hydrogen  star;  but 
owing  to  its  great  brilliancy  and  to  the  fact  that  it  does 
not  attain  a  great  height  above  the  horizon  in  our  lati- 


M  ARCH— CAN  is  MAJOR 


30   ASTRONOMY  FOR  YOUNG  FOLKS 

tudes,  its  rays  are  greatly  refracted  or  broken  up  by 
the  atmosphere,  which  is  most  dense  near  the  horizon, 
and  as  a  result,  it  twinkles  or  scintillates  more  noticeably 
than  other  stars  and  flashes  the  spectrum  colors — 
chiefly  red  and  green — like  a  true  "diamond  in  the 
sky" — a  magnificent  object  in  the  telescope. 

Sirius  is  one  of  our  nearest  neighbors  among  the 
stars.  Only  two  stars  are  known  to  be  nearer  to  the 
solar  system.  Yet  its  light  takes  about  eight  and  a  half 
years  to  flash  with  lightning  speed  across  the  great 
intervening  chasm.  It  is  attended  also  by  a  very  faint 
star  that  is  so  lost  in  the  rays  of  its  brilliant  companion 
that  it  can  only  be  found  with  the  aid  of  a  powerful 
telescope.  The  two  stars  are  separated  by  a  distance 
of  1,800,000,000  miles;  that  is  they  are  about  as  far 
apart  as  Neptune  and  the  sun.  They  swing  slowly 
and  majestically  about  a  common  center,  called  their 
center  of  gravity,  in  a  period  of  about  forty -nine  years. 
So  faint  is  the  companion  of  Sirius  that  it  is  estimated 
that  twenty  thousand  such  stars  would  be  needed  to 
give  forth  as  much  light  as  Sirius.  The  two  stars 
together,  Sirius  and  its  companion,  give  forth  twenty- 
six  times  as  much  light  as  our  own  sun.  They  weigh 
only  about  three  times  as  much,  however.  The  com- 
panion of  Sirius,  in  spite  of  its  extreme  faintness, 
weighs  fully  half  as  much  as  the  brilliant  star. 

There  are  a  number  of  bright  stars  in  the  constella- 
tion of  Canis  Major.  A  fairly  bright  star  a  little  to 
the  west  of  Sirius  marks  the  uplifted  paw  of  the  dog, 


MARCH  31 

and  to  the  southeast,  in  the  tail  <and  hind  quarters,  are 
several  conspicuous  stars  of  the  second  magnitude. 

A  little  to  the  east  and  much  farther  to  the  north, 
we  find  Canis  Minor,  The  Lesser  Dog,  containing  the 
beautiful  first-magnitude  star  Procyon,  "Precursor  of 
the  Dog" — that  is,  of  Sirius.  Since  Procyon  is  so 
much  farther  north  than  Sirius  and  very  little  to  the 
east,  we  see  its  brilliant  rays  in  the  eastern  sky  some 
time  before  Sirius  appears  above  the  southeastern 
horizon,  hence  its  name.  Long  after  Sirius  has  dis- 
appeared from  view  beneath  the  western  horizon  in  the 
late  spring,  Procyon  may  still  be  seen  low  in  the 
western  sky.  Procyon,  also  is  one  of  our  nearer 
neighbors  among  the  stars,  being  only  about  ten  light- 
years  distant  from  the  solar  system.  Like  Sirius,  it 
is  a  double  star  with  a  much  fainter  companion,  that 
by  its  attraction  sways  the  motion  of  Procyon  to  such 
an  extent  that  we  should  know  of  its  existence,  even 
if  it  were  not  visible,  by  the  disturbances  it  produces 
in  the  motion  of  Procyon.  The  period  of  revolution  of 
Procyon  and  its  companion  about  a  common  center  is 
about  forty  years,  and  the  two  stars  combined  weigh 
about  a  third  more  than  our  own  sun  and  give  forth 
six  times  as  much  light.  Canis  Minor  contains  only 
one  other  bright  star,  Beta,  a  short  distance  to  the 
northwest  of  Procyon.  Originally,  the  name  Procyon 
was  given  to  the  entire  constellation,  but  it  was  later 
used  only  with  reference  to  the  one  star.  Procyon, 
Sirius,  and  Betelgeuze  in  Orion  form  a  huge  equal- 
sided  triangle  that  lies  across  the  meridian  at  this  time 


32        ASTRONOMY  FOR  YOUNG  FOLKS 

and  is  a  most  conspicuous  configuration  in  the  evening 
sky. 

Directly   south  of   the  zenith   we   find   Gemini,   The 
Twins,   one   of   the   zodiacal   constellations.      It    is    in 


MARCH — GEMINI  AND  CANIS  MINOR 

Gemini  that  the  sun  is  to  be  found  at  the  beginning  of 
summer.  The  two  bright  stars  Castor  and  Pollux 
mark  the  heads  of  the  twins,  and  the  two  stars  in  the 
opposite  corners  of  the  four-sided  figure  shown  in  the 
chart  mark  their  feet. 

Castor   and   Pollux,    according   to   the    legend,    were 
the   twin   brothers    of    Helen    of   Troy   who    went   on 


MARCH  33 

the  Argonautic  expedition.  When  a  storm  overtook 
the  vessel  on  its  return  voyage,  Orpheus  invoked  the 
aid  of  Apollo,  who  caused  two  stars  to  shine  above 
the  heads  of  the  twins,  and  the  storm  immediately 
ceased.  It  was  for  this  reason  that  Castor  and  Pollux 
became  the  special  deities  of  seamen,  and  it  was  custom- 
ary to  place  their  effigies  upon  the  prows  of  vessels. 
The  "By  Jimini!"  of  today  is  but  a  corruption  of 
the  "By  Gemini!"  heard  so  frequently  among  the  sailors 
of  the  ancient  world. 

The  astronomical  name  for  Castor,  the  fainter  star, 
is  Alpha  Geminorum,  meaning  Alpha  of  Gemini.  As 
it  was  customary  to  call  the  brightest  star  in  a  con- 
stellation by  the  first  letter  in  the  Greek  alphabet,  it 
is  believed  that  Castor  has  decreased  considerably  in 
brightness  since  the  days  of  the  ancients,  for  it  is  now 
decidedly  inferior  to  Pollux  in  brightness,  which  is 
called  Beta  Geminorum.  Of  the  two  stars,  Castor  is 
the  more  interesting  because  it  is  a  double  star  that 
is  readily  separated  into  two  stars  with  the  aid  of  a 
small  telescope.  The  two  principal  stars  are  known  to 
be,  in  turn,  extremely  close  double  stars  revolving  al- 
most in  contact  in  periods  of  a  few  days.  Where  we 
see  but  one  star  with  the  unaided  eye,  there  is,  then 
a  system  of  four  suns,  the  two  close  pairs  revolving* 
slowly  about  a  common  center  of  gravity  in  a  period 
of  several  centuries  and  at  a  great  distance  apart. 

The  star  Pollux,  which  we  can  easily  distinguish  by 
its  superior  brightness,  is  the  more  southerly  of  the 


34        ASTRONOMY  FOR  YOUNG  FOLKS 

twin  stars  and  lies  due  north  of  Procyon  and  about  as 
far  from  Procyon  as  Procyon  is  from  Sirius. 

The  appearance  of  Gemini  on  the  meridian  in  the 
early  evening  and  of  the  huge  triangle,  with  its  corners 
marked  by  the  brilliants,  Procyon,  Sirius,  and  Betel- 
geuze,  due  south,  with  "Great  Orion  sloping  slowly 
to  the  west,"  is  as  truly  a  sign  of  approaching  spring 
as  the  gradual  lengthening  of  the  days,  the  appearance 
of  crocuses  and  daffodils,  and  the  first  robin.  It  is 
only  a  few  weeks  later — as  pictured  by  Tennyson  in 
Maud — 

"When  the  face  of  the  night  is  fair  on  the  dewy  downs, 
And  the  shining  daffodil  dies,  and  the  Charioteer 
And  starry  Gemini  hang  like  glorious  crowns 
Over  Orion's  grave  low  down  in  the  west." 


APRIL 

IN  the  early  evening  hours  of  April  the  western  sky 
is  still  adorned  with  the  brilliant  jewels  with  which 
we  became  familiar  on  the  clear  frosty  evenings  of 
winter.  Orion  is  now  sinking  fast  to  his  rest  beneath 
the  western  horizon.  Beautiful,  golden  Capella  in 
Auriga  glows  in  the  northwest.  Sirius  sparkles  and 
scintillates,  a  magnificent  diamond  of  the  sky,  just 
above  the  southwestern  horizon,  while  Procyon  in 
Canis  Minor,  The  Lesser  Dog,  and  Castor  and  Pollux, 
The  Twins,  in  the  constellation  of  Gemini,  are  still 
high  in  the  western  part  of  the  heavens. 

In  the  northeast  and  east  may  be  seen  the  con- 
stellations that  will  be  close  to  the  meridian  at  this 
time  next  month.  Ursa  Major,  The  Greater  Bear, 
with  its  familiar  Big  Dipper,  is  now  in  a  favorable 
position  for  observation.  The  Sickle  in  Leo  is  high  in 
the  eastern  sky,  and  Spica,  the  brilliant  white  diamond 
of  the  evening  skies  of  spring,  is  low  in  the  southeast 
in  Virgo. 

Near  the  meridian  this  month  we  find  between  Auriga 
and  Ursa  Major,  and  east  of  Gemini,  the  inconspicuous 
constellation  of  Lynx,  which  contains  not  a  single 
bright  star  and  is  a  modern  constellation  devised 

35 


36   ASTRONOMY  FOR  YOUNG  FOLKS 

simply  to   fill   the  otherwise  vacant   space  in  circum- 
polar  regions  between  Ursa  Major  and  Auriga. 

Just   south   of    the   zenith   at   this   time,   and   lying 
between  Gemini   and  Leo,  is   Cancer,  The  Crab,  the 


APRIL — CANCER 

most  inconspicuous  of  all  the  zodiacal  constellations. 
There  are  no  bright  stars  in  this  group,  and  there  is 
also  nothing  distinctive  about  the  grouping  of  its 
faint  stars,  though  we  can  readily  find  it,  from  its 
position  between  the  two  neighboring  constellations 
of  Gemini  and  Leo  by  reference  to  the  chart. 

In  the  position  indicated  there  we  will  see  on  clear 


APRIL  37 

evenings  a  faint,  nebulous  cloud  of  light,  which  is 
known  as  Praesepe,  The  Beehive,  or  as  The  Manger, 
the  two  faint  stars  flanking  it  on  either  side  being 
called  Aselli,  The  Asses.  This  faint  cloud  can  be 
easily  resolved  by  an  opera-glass  into  a  coarse  cluster 
of  stars  that  lie  just  beyond  the  range  of  the  unaided 
human  vision. 

To  the  ancients,  Praesepe  served  as  an  indicator  of 
weather  conditions,  and  Aratus,  -an  ancient  astronomer, 
wrote  of  this  cluster: 

"A  murky  manger,  with  both  stars 
Shining  unaltered,  is  a  sign  of  rain. 
If  while  the  northern  ass  is  dimmed 
By  vaporous  shroud,  he  of  the  south  gleam  radiant, 
Expect  a  south  wind ;  the  vaporous  shroud  and  radiance 
Exchanging  stars,  harbinger  Boreas." 

This  was  not  entirely  a  matter  of  superstition,  as  we 
might  possibly  imagine,  for  the  dimness  of  the  cluster 
is  simply  an  indication  that  vapor  is  gathering  and 
condensing  in  the  atmosphere,  just  as  a  ring  around 
the  moon  is  an  indication  of  the  same  gathering  and 
condensation  of  vapor  that  precedes  a  storm. 

Some  centuries  ago  the  sun  reached  its  greatest 
distance  north  of  the  equator — which  occurs  each  year 
at  the  beginning  of  summer — at  the  time  when  it 
was  passing  through  the  constellation  of  Cancer.  Our 
tropic  of  Cancer,  which  marks  the  northern  limit  of 
the  torrid  zone,  received  its  name  from  this  fact.  At 
the  time  when  the  sun  reaches  the  point  farthest  north, 
its  height  above  the  horizon  changes  very  little  from 


38 


ASTRONOMY  FOR  YOUNG  FOLKS 


day  to  day,  and  for  a  short  time  it  appears  to  be 
slowly  crawling  sideways  through  the  heavens,  as  a 
crab  walks,  and  for  this  reason,  possibly,  the  con- 
stellation was  called  Cancer,  The  Crab.  At  the  present 


HYiDftA 


C RATER 

••  *•-•' 

I-  .•..»-••* 

'CdRvuS      ',    -'       HYDRA 


APRIL— HYDRA 

time  the  "Precession  of  the  Equinoxes,"  or  westward 
shifting  of  the  vernal  equinox — the  point  where  the 
sun  crosses  the  equator  going  north  in  the  spring — 
brings  the  sun,  when  it  is  farthest  north,  in  Gemini 
instead  of  in  Cancer.  At  the  present  time,  then  it 
would  be  more  accurate  to  speak  of  the  tropic  of 
Gemini,  though  this  in  turn  would  be  inaccurate  after 


APRIL  39 

a  lapse  of  centuries,  as  the  sun  passed  into  another  con- 
stellation at  the  beginning  of  summer.  The  tropic  of 
Capricorn,  which  marks  the  farthest  southern  excur- 
sions of  the  sun  in  its  yearly  circuit  of  the  heavens, 
should  also  more  appropriately  be  called  the  tropic 
of  Sagittarius,  as  the  sun  is  now  in  Sagittarius  instead 
of  Capricornus  at  the  time  when  it  is  farthest  south, 
though  the  point  is  slowly  shifting  westward  into 
Scorpio. 

Mythology  tells  us  that  Cancer  was  sent  by  Juno  to 
distract  Hercules  by  pinching  his  toes  while  he  was 
contending  with  the  many-headed  serpent  in  the  Lern- 
ean  swamp.  Hercules,  the  legend  says,  crushed  the 
crab  with  a  single  blow,  and  Juno  by  way  of  reward 
placed  it  in  the  heavens. 

In  Cancer,  according  to  the  belief  of  the  Chaldeans, 
was  located  the  "gate  of  men,"  by  which  souls 
descended  into  human  bodies,  while  in  Capricornus 
was  the  "gate  of  the  gods,"  through  which  the  freed 
souls  of  men  returned  to  heaven. 

Hydra,  the  many-headed  serpent  with  which  Hercu- 
les contended,  is  represented  by  a  constellation  of 
great  length.  It  extends  from  a  point  just  south  of 
Cancer,  where  a  group  of  faint  stars  marks  the  heads, 
to  the  south  and  southeast  in  a  long  line  of  faint 
stars.  It  passes  in  its  course  just  south  of  Crater  and 
Corvus,  the  two  small  star-groups  below  Leo  (see 
constellations  for  May),  which  are  sometimes  called 
its  riders,  and  it  also  stretches  below  the  entire  length 
of  the  long,  straggling  constellation  of  Virgo.  At  this 


40        ASTRONOMY  FOR  YOUNG  FOLKS 

time  we  can  trace  it  only  to  the  point  where  it  dis- 
appears below  the  horizon  in  the  southeast.  It  con- 
tains but  one  bright  star,  Alphard,  or  Cor  Hydrae  as 
it  is  also  called,  standing  quite  alone  and  almost  due 


APRIL — LYNX 

south  at  this  time.  Hydra,  as  well  as  Lynx  and  Cancer, 
contains  no  noteworthy  or  remarkable  object  and  con- 
sists chiefly  of  faint  stars.  Alphard  is,  in  fact,  the 
only  bright  star  that  we  have  in  the  constellations  for 
this  month.  It  chances  that  these  three  inconspicuous 
star-groups,  Lynx,  Crater,  and  Hydra,  lie  nearest  to 
the  meridian  at  this  time,  separating  the  brilliant  groups 
of  winter  from  those  of  the  summer  months. 


VI 

MAY 

URSA  Major,  the  Great  Bear,  and  Ursa  Minor,  the 
Lesser  Bear,  or,  as  they  are  more  familiarly  called, 
the  Big  Dipper  and  the  Little  Dipper,  are  the  best 
known  of  all  the  constellations  visible  in  northern  lati- 
tudes. They  are  called  circumpolar  constellations,  which 
means  "around  the  pole."  For  those  who  live  north  of 
40°  N.  Lat.  they  never  set,  but  can  be  seen  at  all  hours 
of  the  night  and  at  all  times  of  the  year.  In  fall  and 
winter  evenings  Ursa  Major  lies  below  the  pole  and 
near  the  horizon,  and  so  is  usually  hidden  more  or 
less  from  view  by  trees  or  buildings.  It  is  during  the 
early  evening  hours  of  late  spring  and  summer  that 
this  constellation  is  seen  to  the  best  advantage  high  in 
the  sky  above  the  pole.  If  one  looks  due  north  at 
the  time  mentioned,  it  will  be  impossible  to  miss  either 
of  these  constellations. 

To  complete  the  outline  of  the  Great  Bear,  it  is  neces- 
sary to  include  faint  stars  to  the  east,  which  form  the 
head  of  the  Bear,  and  other  faint  stars  to  the  south, 
which  form  the  feet,  but  these  are  all  inconspicuous 
and  of  little  general  interest. 

The  two  stars  in  the  bowl  of  the  Big  Dipper 
through  which  an  arrow  is  drawn  in  the  chart,  are 

41 


42        ASTRONOMY  FOR  YOUNG  FOLKS 

called  the  Pointers,  because  an  imaginary  line  drawn 
through  these  two  stars  and  continued  a  distance  about 
equal  to  the  length  of  the  Big  Dipper,  brings  us  to 
the  star  Polaris,  or  the  North  Star,  at  the  end  of  the 


MAY— URSA  MAJOR  AND  URSA  MINOR 


handle  of  the  Little  Dipper,  which  is  very  close  to 
the  north  pole  of  the  heavens,  the  direction  in  which 
the  earth's  axis  points.  The  pole  lies  on  the  line  con- 
necting the  star  at  the  bend  in  the  handle  of  the  Big 
Dipper  with  Polaris,  and  is  only  one  degree  distant 
from  the  pole-star. 

The  distance  between  the  Pointers  is  five  degrees  of 


MAY  43 

arc,  and  the  distance  from  the  more  northerly  of  these 
two  stars  to  Polaris  is  nearly  thirty  degrees.  We  may 
find  it  useful  to  remember  this  in  estimating  distances 
between  objects  in  the  heavens,  which  are  always  given 
in  angular  measure. 

A  small  two  and  one-half  inch  telescope  will  separate 
Polaris  into  two  stars  eighteen  seconds  of  arc  apart. 
The  companion  star  is  a  faint  white  star  of  the  ninth 
magnitude. 

Twenty  years  or  so  ago  it  was  discovered  with  the 
aid  of  the  spectroscope  that  the  brighter  of  the  two 
stars  was  also  a  double  star,  but  the  two  stars  were 
so  close  together  that  they  could  not  be  seen  as  sepa- 
rate stars  in  any  telescope.  Later  it  was  found  that 
the  brighter  star  was  in  reality  triple,  that  is,  it  con- 
sists of  three  suns  close  together.  The  faint  white 
companion  star  formed  with  these  three  suns  a  system 
of  four  suns  revolving  about  a  common  center  of  grav- 
ity. Still  more  recently  it  has  been  discovered  that  the 
brightest  of  these  four  suns  varies  regularly  in  bright- 
ness in  a  period  of  a  little  less  than  four  days.  It  be- 
longs to  the  important  class  of  stars  known  as  Cepheid 
variable  stars,  whose  changes  of  light,  it  is  believed, 
are  produced  by  some  periodic  form  of  disturbance 
taking  place  within  the  stars  themselves. 

With  one  exception,  Polaris  is  the  nearest  to  the 
earth  of  all  these  Cepheid  variable  stars,  which  are  in 
most  instances  at  very  great  distances  from  the  solar 
system.  The  latest  measurements  of  the  distance  of 
Polaris  show  that  its  light  takes  about  two  centuries 


44        ASTRONOMY  FOR  YOUNG  FOLKS 

to  travel  to  the  earth,   or,   in  other  words,  that  it  is 
distant  two  hundred  light-years. 

Like  all  Cepheid  variables,  Polaris  is  a  giant  star.    It 
gives  forth  about  five  hundred  and  twenty-five  times 


MAY— LEO 

as  much  light  as  our  own  sun.  If  Polaris  and  the  sun 
were  placed  side  by  side  at  a  distance  of  thirty- three 
light-years,  the  sun  would  appear  as  a  star  of  the  fifth 
magnitude,  just  well  within  the  range  of  visibility  of 
the  human  eye,  while  Polaris  would  outshine  Sirius,  the 
brightest  star  in  the  heavens. 

As  a  practical  aid  to  navigators,   Polaris  is  unsur- 


MAY  45 

passed  in  importance  by  any  star  of  the  northern  hemi- 
sphere of  the  heavens.  At  the  equator  the  pole-star 
lies  in  the  horizon;  at  the  north  pole  of  the  earth  it 
is  in  the  zenith  or  directly  overhead.  Its  altitude  or 
height  above  the  horizon  is  always  equal  to  the  latitude 
of  the  place  of  observation.  As  we  travel  northward 
from  the  equator  toward  the  pole  we  see  Polaris  rise 
higher  and  higher  in  the  sky.  In  New  York  the  ele- 
vation of  Polaris  above  the  horizon  is  forty  degrees, 
which  is  the  latitude  of  the  city. 

The  Pointers  indicate  the  direction  of  Polaris  and 
the  true  north,  while  the  height  of  Polaris  above  the 
horizon  tells  us  our  latitude.  These  kindly  stars  direct  us 
by  night  when  we  are  uncertain  of  our  bearings,  whether 
we  travel  by  land  or  sea  or  air.  They  are  the  friends 
and  aids  of  explorers,  navigators  and  aviators,  who 
often  turn  to  them  for  guidance. 

Bryant  writes  thus  beautifully  of  Polaris  in  his 
Hymn  to  the  North  Star: 

Constellations  come  and  climb  the  heavens,  and  go. 
Star  of  the  Pole!  and  thou  dost  see  them  set. 

Alone  in  thy  cold  skies, 
Thou  keep'st  thy  old  unmoving  station  yet, 
Nor  join'st  the  dances  of  that  glittering  train, 
Nor  dipp'st  thy  virgin  orb  in  the  blue  western  main. 

On  thy  unaltering  blaze 
The  half  wrecked  mariner,  his  compass  lost, 

Fixes  his  steady  gaze, 

And  steers,  undoubting,  to  the  friendly  coast; 
And  they  who  stray  in  perilous  wastes  by  night, 
Are  glad  when  thou  dost  shine  to  guide  their  footsteps 
right. 


46        ASTRONOMY  FOR  YOUNG  FOLKS 

The  star  at  the  bend  in  the  handle  of  the  Big  Dipper, 
called  Mizar,  is  of  special  interest.  If  one  has  good  eye- 
sight, he  will  see  close  to  it  a  faint  star.  This  is  Alcor, 
which  is  Arabic  for  The  Test  The  two  stars  are 
also  called  the  Horse  and  the  Rider. 

Mizar  forms  with  Alcor  what  is  known  as  a  wide 
double  star.  It  is,  in  fact,  the  widest  of  all  double 
stars.  Many  stars  in  the  heavens  that  appear  single 
to  us  are  separated  by  the  telescope  into  double  or 
triple  or  multiple  stars.  They  consist  of  two  or  more 
suns  revolving  about  a  common  center,  known  as  their 
center  of  gravity.  Sometimes  the  suns  are  so  close 
together  that  even  the  most  powerful  telescope  will  not 
separate  them.  Then  a  most  wonderful  little  instru- 
ment, called  the  spectroscope,  steps  in  and  analyzes  the 
light  of  the  stars  and  shows  which  are  double  and  which 
are  single.  A  star  shown  to  be  double  by  the  spectro- 
scope, but  not  by  the  telescope,  is  called  a  spectroscopic 
binary  star. 

Mizar  is  of  historic  interest,  as  being  the  first  double 
star  to  be  detected  with  the  aid  of  the  telescope.  A 
very  small  telescope  will  split  Mizar  up  into  two  stars. 
The  brighter  of  the  two  is  a  spectroscopic  binary  star 
beside,  so  that  it  really  consists  of  two  suns  in- 
stead of  one,  with  the  distance  between  the  two  so 
small  that  even  the  telescope  cannot  separate  them. 
About  this  system  of  three  suns  which  we  know  as 
the  star  Mizar,  the  faint  star  Alcor  revolves  at  a  dis- 
tance equal  to  sixteen  thousand  times  the  distance  of 
the  earth  from  the  sun. 


MAY 


47 


If  we  follow  the  imaginary  line  drawn  through  the 
Pointers  in  a  southerly  direction  about  forty-five  de- 
grees, we  come  to  Leo,  The  Lion,  one  of  the  zodiacal 
constellations.  There  should  'be  no  difficulty  in  finding 


MAY — CORVUS  AND  CRATER 

the  constellation  Leo,  as  its  peculiar  sickle-shaped  group 
of  bright  stars  makes  it  distinctive  from  all  other  con- 
stellations. At  the  time  we  have  mentioned,  that  is, 
the  early  evening  hours,  it  will  lie  a  little  to  the  south- 
west of  the  zenith.  Leo  is  one  of  the  finest  constella- 
tions and  is  always  associated  with  the  spring  months 
because  it  is  then  high  in  the  sky  in  the  evening. 


48        ASTRONOMY  FOR  YOUNG  FOLKS 

Regulus  is  the  beautiful  white  star  which  marks  the 
handle  of  The  Sickle,  and  the  heart  of  Leo;  and  Dene- 
bola  is  the  second-magnitude  star  in  the  tail  of  Leo. 
Due  south  of  Denebola,  about  thirty  degrees,  we  find 
the  small  star-group  known  as  Crater,  The  Cup,  which 
is  composed  of  rather  faint  and  inconspicuous  stars. 
Just  east  of  Crater  is  the  group  known  as  Corvus,  The 
Crow,  which  forms  a  very  characteristic  little  four- 
sided  figure  of  stars  differing  very  little  from  one  an- 
other in  brightness.  These  two  star  groups  lie  far 
to  the  south  in  our  latitudes;  but  if  we  lived  twenty  de- 
grees south  of  the  equator,  we  would  find  them  nearly 
overhead,  at  this  time  of  the  year.  Just  south  of 
Corvus  and  Crater  we  find  Hydra,  one  of  the  constel- 
lations for  April  which  extends  beneath  these  constella- 
tions and  also  beneath  Virgo,  one  of  the  June  con- 
stellations. 


VII 
JUNE 

THE  star-groups  that  occupy  the  center  of  the  celestial 
stage  in  mid-latitudes  of  the  northern  hemisphere 
during  the  early  evening  hours  of  June  are  Bootes, 
often  called  The  Hunter,  (although  the  word  means 
Herdsman  or  Shouter,  which  will  be  found  overhead 
at  this  time;  Virgo,  The  Maiden,  largest  of  the  zodiacal 
constellations,  lying  nearly  due  south;  Canes  Venatici, 
The  Hunting  Dogs;  Corona  Borealis,  The  Northern 
Crown,  and  Coma  Berenices. 

The  gorgeous  orange-hued  Arcturus  in  Bootes  and 
the  beautiful  bluish-white  Spica  in  Virgo,  like  a  dia- 
mond in  its  sparkling  radiance,  form  with  Denebola  in 
Leo,  which  we  identified  in  May,  a  huge  equal-sided 
triangle  that  is  always  associated  with  the  spring  and 
early  summer  months. 

To  the  west  of  Bootes,  below  the  handle  of  the  Big 
Dipper,  is  a  region  where  there  are  few  conspicuous 
stars.  Here  will  be  found  Canes  Venatici  (The  Hunt- 
ing Dogs  with  which  Bootes  is  supposed  to  be  pursu- 
ing the  Great  Bear  around  the  north  pole),  and,  fur- 
ther south,  Coma  Berenices  (Bernice's  Hair). 

The  brighter  of  the  two  Hunting  Dogs,  which  is  also 
the  brightest  star  in  the  entire  region  covered  by  these 
two  constellations,  appears  as  a  beautiful  blue-and-yellow 

49 


50        ASTRONOMY  FOR  YOUNG  FOLKS 

double  star  in  the  telescope.  It  was  named  Cor  Caroli 
(Heart  of  Charles)  by  the  astronomer  Halley  in  honor 
of  Charles  II  of  England,  at  the  suggestion  of  the 
court  physician,  who  imagined  it  shone  more  brightly 
than  usual  the  night  before  the  return  of  Charles  to 
London.  Of  more  interest  to  astronomers  is  the  mag- 
nificent spiral  nebula  in  this  constellation,  known  as 
the  "Whirlpool  Nebula,"  appearing  as  a  faint,  luminous 
patch  in  the  sky,  of  which  many  photographs  have  been 
taken  with  the  great  telescopes.  This  entire  region, 
from  Canes  Venatici  to  Virgo,  abounds  in  faint  spiral 
nebulae  that  for  some  reason  not  yet  understood  by 
astronomers  are  crowded  together  in  this  part  of  the 
heavens  where  stars  are  comparatively  few.  It  is  be- 
lieved that  there  are  between  five  hundred  thousand  and 
a  million  of  these  spiral  nebulae  in  the  entire  heavens, 
and  the  problem  of  their  nature  and  origin  and  dis- 
tance is  one  that  the  astronomers  are  very  anxious  to 
solve.  Many  wonderful  facts  are  now  being  learned 
concerning  these  faint  nebulous  wisps  of  light  which, 
with  a  few  exceptions,  are  observable  only  with  great 
telescopes.  They  reveal  their  spiral  structure  more 
clearly  to  the  photographic  plate  than  to  the  human 
eye,  and  some  magnificent  photographs  of  them  have 
been  taken  with  powerful  telescopes. 

Coma  Berenices,  south  of  Canes  Venatici  and  south- 
west of  Bootes,  is  a  constellation  that  consists  of  a 
great  number  of  stars  closely  crowded  together,  and 
just  barely  visible  to  the  unaided  eye.  As  a  result,  it 
has  the  appearance  of  filmy  threads  of  light,  which 


JUNE  51 

doubtless  suggested  its  name  to  the  imaginative  an- 
cients, who  loved  to  fill  the  heavens  with  fanciful  crea- 
tions associated  with  their  myths  and  legends.  These 
stars  form  a  moving  cluster  of  stars  estimated  to  be 


CANES 

BOOTi-S  VENA'T 


. 

BOREAUS 


JUNE— BOOTES.  CANES  VENATICI  AND  COMA  BERENICIS 

at  a  distance  of  about  270  light-years  from  the  solar 
systems. 

This  region,  so  lacking  in  interesting  objects  for  the 
naked-eye  observer,  is  a  mine  of  riches  to  the  fortunate 
possessors  of  telescopes;  and  the  great  telescopes  of  the 
world  are  frequently  pointed  in  this  direction,  explor- 
ing the  mysteries  of  space  that  abound  here. 


52        ASTRONOMY  FOR  YOUNG  FOLKS 

Just  to  the  east  of  Bootes  is  the  exquisite  little  circlet 
of  stars  known  as  Corona  Borealis,  the  Northern  Crown. 
It  consists  of  six  stars  arranged  in  a  nearly  perfect 
semicircle,  and  one  will  have  no  difficulty  in  recogniz- 
ing it.  Its  brightest  star,  Alpha,  known  also  by  the 
name  of  Alphacca,  is  a  star  of  the  second  magnitude. 

Bootes  is  one  of  the  largest  and  finest  of  the  northern 
constellations.  It  can  be  easily  distinguished  by  its 
peculiar  kite-shaped  grouping  of  stars  or  by  the  con- 
spicuous pentagon  (five-sided  figure)  of  stars  which  it 
contains.  The  most  southerly  star  in  this  pentagon  is 
known  as  Epsilon  Bootes  and  is  one  of  the  finest  double 
stars  in  the  heavens.  The  two  stars  of  which  it  con- 
sists are  respectively  orange  and  greenish-blue  in  color. 

By  far  the  finest  object  in  Bootes,  however,  is  the 
magnificent  Arcturus,  which  is  the  brightest  star  in  the 
northern  hemisphere  of  the  heavens.  This  star  will 
be  conspicuous  in  the  evening  hours  throughout  the 
summer  months,  as  will  also  the  less  brilliant  Spica  in 
Virgo. 

Some  recent  measurements  show  that  Arcturus  is 
one  of  our  nearer  neighbors  among  the  stars.  Its  dis- 
tance is  now  estimated  to  be  about  twenty-one  light- 
years.  That  is,  a  ray  of  light  from  this  star  takes 
twenty-one  years  to  reach  the  earth,  traveling  at  the  rate 
of  one  hundred  and  eighty-six  thousand  miles  per 
second.  It  would  seem  as  if  we  should  hardly  speak  of 
Arcturus,  twenty-one  light-years  away,  as  a  near  neigh- 
bor, yet  there  are  millions  of  stars  that  are  far  more 


JUNE 


53 


distant  from  the  earth,  and  very  few  that  are  nearer 
to  us  than  Arcturus. 

The  brightness  of  Arcturus  is  estimated  to  be  about 
forty   times  that  of   the   sun.      That   is,   if   the   two 


JUNE — VIRGO 

bodies   were  side   by   side,   Arcturus   would   give    forth 
forty  times  as  much  light  and  heat  as  the  sun. 

Arcturus  is  also  one  of  the  most  rapidly  moving  stars 
in  the  heavens.  In  the  past  sixteen  centuries  it  has 
traveled  so  far  as  to  have  changed  its  position  among 
the  other  stars  by  as  much  as  the  apparent  width  of 
the  moon.  Most  of  the  stars,  in  spite  of  their  motions 


54        ASTRONOMY  FOR  YOUNG  FOLKS 

through  the  heavens  in  various  directions,  appear  to- 
day in  the  same  relative  positions  in  which  they  were 
several  thousand  years  ago.  It  is  for  this  reason  that 
the  constellations  of  the  Egyptians  and  of  the  Greeks 
and  Romans  are  the  same  constellations  that  we  see 
in  the  heavens  today.  Were  all  the  stars  as  rapidly 
moving  as  Arcturus,  the  distinctive  forms  of  the  con- 
stellations would  be  preserved  for  only  a  very  few 
centuries. 

Virgo,  which  lies  south  and  southwest  of  Bootes,  is 
a  large,  straggling  constellation,  consisting  of  a  Y-shaped 
configuration  of  rather  inconspicuous  stars.  It  lies  in 
the  path  of  our  sun,  moon  and  planets,  and  so  is  one 
of  the  zodiacal  constellations.  The  cross  in  the  dia- 
gram indicates  the  present  position  of  the  autumnal 
equinox,  the  point  where  the  sun  crosses  the  equator 
going  south,  and  the  position  the  sun  now  occupies 
at  the  beginning  of  fall. 

Spica,  the  brightest  star  in  Virgo,  is  a  bluish-white, 
first-magnitude  star,  standing  very  much  alone  in  the 
sky.  In  fact,  the  Arabs  referred  to  this  star  as  "The 
Solitary  One."  Its  distance  from  the  earth  is  not 
known,  but  must  be  very  great  -as  it  cannot  be  found 
by  the  usual  methods.  The  spectroscope  shows  that 
it  consists  of  two  suns  very  close  together,  revolving 
about  a  common  center  in  a  period  of  only  four  days. 

Within  the  branches  of  the  "Y"  in  Virgo,  and  just 
to  the  north  of  it,  is  the  wonderful  nebulous  region  of 
this  constellation,  but  it  takes  a  powerful  telescope  to 
show  the  faint  spiral  nebulae  that  exist  here  in  great 


JUNE  55 

profusion.  All  of  these  spirals  are  receding  from  the 
plane  of  the  Milky  Way  with  enormous  velocities.  The 
spiral  nebulae  are,  in  fact,  the  most  rapidly  moving 
objects  in  the  heavens. 


VIII 
JULY 

DUE  east  of  the  little  circlet  of  stars  known  as 
Corona  Borealis,  and  almost  directly  overhead 
in  our  latitude  (40°  N.)  about  nine  o'clock  in  the  eve- 
ning in  the  early  part  of  July,  is  the  large  constellation 
of  Hercules,  named  for  the  famous  hero  of  Grecian 
mythology.  There  are  no  stars  of  great  brilliancy  in 
this  group,  but  it  contains  a  large  number  of  fairly 
bright  stars  arranged  in  the  form  outlined  in  the  chart. 
The  hero  is  standing  with  his  head,  marked  by  the 
star  Alpha  Herculis,  toward  the  south,  and  his  foot 
resting  on  the  head  of  Draco,  The  Dragon,  a  far-north- 
ern constellation  with  which  we  become  acquainted  in 
August. 

Alpha  Herculis,  the  best  known  star  in  this  con- 
stellation, is  of  unusual  interest.  Not  only  is  it  a 
most  beautiful  double  star,  the  brighter  of  the  two 
stars  of  which  it  is  composed  being  orange,  and  the 
fainter  greenish-blue,  but  it  is  also  a  star  that  changes 
in  brightness  irregularly.  Both  the  orange  and  the  blue 
star  share  in  this  change  of  brightness.  There  are  a 
number  of  stars  in  ihe  heavens  that  vary  in  bright- 
ness, some  in  very  regular  periods,  and  others,  like 
Alpha  Herculis,  irregularly.  These  latter  stars  are 

56 


THE  GREAT  HERCULES  CLUSTER— A  UNIVERSE  OF  SUNS 
Taken  with  6o-inch  Reflector  of  the  Mt.  Wilson  Observatory 


JULY 


57 


nearly  always  deep  orange  or  reddish  in  color.  One 
may  note  this  variation  in  the  brightness  of  Alpha  Her- 
culis  by  comparing  it  from  time  to  time  with  some 
nearby  star  that  does  not  vary  in  brightness. 


JULY — HERCULES 

The  constellation  of  Hercules  is  a  very  rich  field 
for  the  possessor  of  even  a  small  telescope.  Here  are 
to  be  found  beautifully  colored  double  stars  in  pro- 
fusion, and,  in  addition,  two  remarkable  clusters  of 
stars.  The  brighter  of  the  two  is  known  as  the  Great 
Hercules  Cluster.  Its  position  is  shown  on  the  chart, 
and,  under  favorable  conditions — that  is,  on  a  clear, 


58        ASTRONOMY  FOR  YOUNG  FOLKS 

dark  night,  when  there  is  no  moonlight — it  may  be 
seen  without  the  aid  of  <a  telescope  as  a  small,  faint 
patch  of  light.  One  would  never  suspect  from  such  a 
view  what  a  wonderful  object  this  cluster  becomes: 
when  seen  with  the  aid  of  a  powerful  telescope.  Photo- 
graphs taken  with  the  great  telescopes  show  this  faint 
wisp  of  light  as  a  magnificent  assemblage  of  thousands 
of  stars,  each  a  sun  many  times  more  brilliant  than 
our  own  sun.  The  crowded  appearance  of  the  stars 
in  the  cluster  is  due  partly  to  the  fact  that  it  is  very 
distant  from  the  earth,  though  neighboring  stars  in 
the  cluster  are  indeed  much  nearer  to  one  another  than 
are  the  stars  in  the  vicinity  of  our  solar  system.  It 
has  been  found  that  this  cluster  is  so  far  away  that 
its  light  takes  over  thirty-six  thousand  years  to  reach 
the  earth.  At  the  distance  of  this  cluster,  a  sun  equal 
in  brightness  to  our  own  sun  would  be  so  faint  that  the 
must  powerful  telescope  in  the  world  would  not  show 
it.  So  we  know  that  the  stars  that  are  visible  in  the 
Hercules  cluster  are  far  more  brilliant  than  our  sun. 
A  fair-sized  telescope  will  show  about  four  thousand 
stars  in  this  cluster,  but  the  greatest  telescopes  show 
over  one  hundred  thousand  in  it,  and  there  are  with- 
out doubt  many  more  too  faint  to  be  seen  at  all.  The 
Hercules  cluster  is  called  a  globular  star-cluster,  be- 
cause the  stars  in  it  are  arranged  nearly  in  the  form 
of  a  sphere.  There  are  in  the  heavens  about  ninety 
such  clusters  whose  distances  have  been  found,  and  they 
are  among  the  most  distant  of  all  objects.  Most  of 
them  are  very  faint,  and  a  few  are  over  two  hundred 


JULY 


59 


thousand  light-years  distant  from  the  earth.  The  Hercules 
cluster  is  one  of  the  nearest  and  is  the  most  noted  of  all  of 
these  globular  clusters.  It  is  considered  to  be  one  of  the 
finest  objects  in  the  heavens.  The  other  cluster  in  Her- 


JULY — OPHIUCHUS  AND  SERPENS 

cules   is  also  very  fine,  but  not  to  be  compared  with 
this  one. 

Just  to  the  south  of  Hercules  are  two  constellations, 
Ophiuchus,  The  Serpent-Bearer,  and  Serpens,  The  Ser- 
pent, which  are  so  intermingled  that  it  is  difficult  to  dis- 
tinguish them.  There  are  in  these  two  constellations, 
as  in  Hercules,  no  stars  of  unusual  brilliancy,  but  a 


60        ASTRONOMY  FOR  YOUNG  FOLKS 

large  number  of  fairly  bright  stars.  The  brightest  star 
in  Ophiuchus  is  known  as  Alpha  Ophiuchi  and  it  marks 
the  head  of  the  Serpent-Bearer.  The  two  stars,  Alpha 
Ophiuchi  and  Alpha  Herculis,  are  close  together,  being 
separated  by  a  distance  about  equal  to  that  between  the 
Pointers  of  the  Big  Dipper.  Alpha  Ophiuchi  is  the 
brighter  of  the  two,  and  it  is  farther  east. 

Ophiuchus,  according  to  one  legend,  was  once  a 
physician  on  earth,  and  was  so  successful  as  a  healer 
that  he  could  raise  the  dead.  Pluto,  the  god  of  the 
lower  world,  became  alarmed  for  fear  his  kingdom 
would  become  depopulated,  and  persuaded  Jupiter  to 
remove  Ophiuchus  to  a  heavenly  abode,  where  he  would 
be  less  troublesome.  The  serpent  is  supposed  to  be  a 
symbol  of  his  healing  powers.  The  head  of  Serpens 
is  marked  by  a  group  of  faint  stars  just  south  of 
Corona  Borealis  and  southwest  of  Hercules.  From  here 
a  line  of  fairly  bright  stars  marks  the  course  of  Ser- 
pens southward  to  the  hand  of  Ophiuchus.  Two  stars 
close  together  and  nearly  equal  in  brightness  mark  the 
hand  with  which  the  hero  grasps  the  body  of  the  ser- 
pent. The  other  hand  is  marked  by  an  equally  bright 
single  star  some  distance  to  the  eastward  where  the 
two  constellations  again  meet.  Ophiuchus  is  thus  rep- 
resented as  holding  the  serpent  with  both  hands.  It 
is  not  an  easy  matter  to  make  out  the  outlines  of  these 
straggling  groups,  but  there  are  in  them  several  pairs 
of  stars  nearly  equal  in  brightness  and  about  as  evenly 
spaced  as  the  two  stars  in  the  one  hand  of  Ophiuchus, 


JULY  61 

and  these,  as  well  as  the  diagram,  will  be  of  aid  in 
tracing  the  two  groups. 

Just  south  of  Serpens  and  Ophiuchus  lies  one  of  the 
most  beautiful  and  easily  recognized  constellations  in 
the  heavens.  This  is  the  constellation  of  Scorpio,  The 
Scorpion,  which  will  be  found  not  far  above  the  south- 
ern horizon  at  this  time.  The  small  constellation  of 
Libra,  The  Scales,  which  lies  just  to  the  northwest  of 
Scorpio,  was  at  one  time  a  part  of  this  constellation 
and  represented  the  creature's  claws,  but  some  cen- 
turies ago  its  name  was  changed  to  Libra.  Both 
Scorpio  and  Libra  are  numbered  among  the  twelve 
zodiacal  constellations — that  is,  they  lie  along  the  eclip- 
tic, or  apparent  yearly  path  of  the  sun  among  the  stars. 
Scorpio  is  the  most  brilliant  and  interesting  of  all  the 
zodiacal  groups.  The  heart  of  the  Scorpion  is  marked 
by  the  magnificent  first-magnitude  star  Antares,  which 
is  of  a  deep  reddish  color.  The  name  signifies  Rival 
of  Ares  (Mars).  It  is  so  called  because  it  is  the  one 
star  in  the  heavens  that  most  closely  resembles  Mars, 
and  it  might  be  mistaken  for  the  ruddy  planet  if  one 
were  not  familiar  with  the  constellations.  At  times, 
when  Mars  is  at  a  considerable  distance  from  the 
earth,  it  is  almost  equal  in  brightness  and  general  ap- 
pearance to  this  glowing  red  star  in  the  heart  of  the 
Scorpion.  In  its  trips  around  the  sun,  Mars  passes 
occasionally  very  close  to  Antares,  and  the  two  then 
present  a  very  striking  appearance. 

With  a  telescope  of  medium  size,  one  will  find  an 
exquisite  little  green  companion-star  close  to  Antares. 


62        ASTRONOMY  FOR  YOUNG  FOLKS 

The  little  companion  is  so  close  to  Antares  that  it  is 
difficult  to  find  it  in  the  glare  of  light  from  its  more 
brilliant  neighbor.  Antares  is  one  of  the  giant  stars 
of  the  universe.  In  fact  it  is,  so  far  as  we  know,  the 


JULY — LIBRA  AND  SCORPIO 

greatest  of  all  the  giants.  Its  diameter  is  more  than 
five  hundred  times  that  of  our  own  sun  and  nearly 
twice  that  of  the  giant  star  Betelgeuze  in  Orion.  If 
placed  at  the  center  of  the  solar  system  its  surface 
would  lie  far  beyond  the  orbit  of  Mars. 

Both    Ophiuchus    and    Scorpio    are    crossed    by    the 
Milky  Way,  that  broad  belt  of  numberless  faint  stars 


JULY  63 

that  encircles  the  heavens.  Some  of  the  most  wonder- 
ful and  beautiful  regions  of  the  Milky  Way  are  to  be 
found  in  these  two  constellations. 

At  various  times  in  the  past,  there  have  suddenly 
flashed  forth  brilliant  stars  in  the  Milky  Way  which  are 
known  as  "temporary  stars,"  or  "novae."  These  out- 
bursts signify  that  some  celestial  catastrophe  has  taken 
place,  the  nature  or  cause  of  which  is  not  clearly  under- 
stood. Some  of  the  most  brilliant  of  these  outbursts 
have  occurred  in  these  two  constellations.  The  life  of 
a  nova  is  very  short,  a  matter  of  a  few  months,  and  it 
rapidly  sinks  into  oblivion,  so  nothing  is  to  be  seen  of 
some  of  the  most  brilliant  of  all  these  stars  that  have 
appeared  in  this  region  in  the  past.  A  few  are  still 
faintly  visible  in  large  telescopes. 


IX 
AUGUST 

IT  was  one  of  the  twelve  labors  of  Hercules,  the 
hero  of  Grecian  mythology,  to  vanquish  the  dragon 
that  guarded  the  golden  apples  in  the  garden  of  the 
Hesperides.  Among  the  constellations  for  July  we 
found  the  large  group  of  stars  that  represents  the  hero 
himself,  and  this  month  we  find  just  to  the  north  of 
Hercules  the  head  of  Draco,  The  Dragon.  The  foot  of 
the  hero  rests  upon  the  dragon's  head,  which  is  outlined 
by  a  group  of  four  fairly  bright  stars  forming  a  quadri- 
lateral or  four-sided  figure.  The  brightest  star  in  this 
group  passes  in  its  daily  circuit  of  the  pole  almost 
through  the  zenith  of  London.  That  is,  as  it  crosses 
the  meridian  of  London,  it  is  almost  exactly  overhead. 
From  the  head  of  Draco,  the  creature's  body  can  be 
traced  in  a  long  line  of  stars  curving  first  eastward, 
then  northward,  toward  the  pole-star  to  a  point  above 
Hercules,  where  it  bends  sharply  westward.  The  body 
of  the  monster  lies  chiefly  between  its  head  and  the  bowl 
of  the  Little  Dipper.  The  tail  extends  in  a  long  line 
of  faint  stars  midway  between  the  two  Dippers,  or  the 
constellations  of  Ursa  Major  and  Ursa  Minor,  the  tip 
of  the  tail  lying  on  the  line  connecting  the  Pointers 
of  the  Big  Dipper  with  the  pole-star  Polaris. 

64 


AUGUST  65 

Draco,  as  well  as  Ursa  Major  and  Ursa  Minor,  is  a  cir- 
cumpolar  constellation  in  our  latitude;  that  is,  it  makes 
its  circuit  of  the  pole  without  at  any  time  dipping  be- 
low the  horizon  in  latitudes  north  of  40°.  It  is,  there- 


•DRACO  * 


LYRA;-*      L v  HERCULES 


AUGUST — DRACO  AND  LYRA 

fore,  visible  at  all  hours  of  the  night  in  mid-latitudes 
of  the  northern  hemisphere,  but  is  seen  to  the  best 
advantage  during  the  early  evening  hours  in  the  sum- 
mer months.  There  are  no  remarkable  stars  in  this  con- 
stellation with  the  exception  of  Alpha,  which  lies  half- 
way between  the  bowl  of  the  Little  Dipper  and  Mizar, 
the  star  at  the  bend  in  the  handle  of  the  Big  Dipper. 


66        ASTRONOMY  FOR  YOUNG  FOLKS 

About  four  thousand  seven  hundred  years  ago,  this 
star  was  the  pole-star — lying  even  nearer  to  what  was 
then  the  north  pole  of  the  heavens  than  Polaris  does  to 
the  present  position  of  the  pole.  The  sun  and  moon 
exert  a  pull  on  the  bulging  equatorial  regions  of  the 
earth,  which  tends  to  draw  the  plane  of  the  earth's 
equator  down  into  the  plane  of  the  ecliptic.  This 
causes  the  "Precession  of  the  Equinoxes"  and  at  the 
same  time  a  slow  revolution  of  the  earth's  axis  of  rota- 
tion about  the  pole  of  the  ecliptic.  The  north  pole 
of  the  heavens  as  a  result  describes  a  circle  about  the 
pole  of  the  ecliptic  of  radius  23/^2°  in  a  period  of 
25,800  years. 

Each  bright  star  that  lies  near  the  circumference  of 
this  circle  becomes  in  turn  the  pole-star  sometime  with- 
in this  period.  The  star  Alpha,  in  Draco,  had  its  turn 
at  being  pole-star  some  forty-seven  centuries  ago. 
Polaris  is  now  a  little  over  a  degree  from  the  north 
pole  of  the  heavens.  During  the  next  two  centuries 
it  will  continue  to  approach  the  pole  until  it  comes 
within  a  quarter  of  a  degree  of  it,  when  its  distance 
from  the  pole  will  begin  to  increase  again.  About 
twelve  thousand  years  hence  the  magnificent  Vega, 
whose  acquaintance  we  will  now  make,  will  be  the 
most  brilliant  and  beautiful  of  all  pole-stars. 

Vega  (Arabic  for  "Falling  Eagle")  is  the  resplendent, 
bluish-white,  first-magnitude  star  that  lies  in  the  con- 
stellation of  Lyra,  The  Lyre  or  Harp,  a  small,  but  im- 
portant, constellation  just  east  of  Hercules  and  a  little 
to  the  southeast  of  the  head  of  Draco.  Vega  is  almost 


AUGUST  67 

exactly  equal  in  brightness  to  Arcturus,  the  orange- 
colored  star  in  Bootes,  now  lying  west  of  the  meridian 
in  the  early  evening  hours.  It  is  also  a  near  neighbor 
of  the  solar  system,  its  light  taking  something  like  forty 
years  to  travel  to  the  earth.  Vega  is  carried  nearly 
through  the  zenith  of  Washington  and  all  places  in  the 
same  latitude  by  the  apparent  daily  rotation  of  the 
heavens.  It  is  a  star  that  we  have  no  difficulty  in 
recognizing,  owing  to  the  presence  of  two  nearby  stars 
that  form,  with  it,  a  small  equal-sided  triangle  with 
sides  only  two  degrees  in  extent.  If  our  own  sun  were 
at  the  distance  of  Vega,  it  would  not  appear  as  bright 
as  one  of  these  faint  stars,  so  much  more  brilliant  is 
this  magnificent  sun  than  our  own.  The  two  faint  stars 
that  follow  so  closely  after  Vega  and  form  the  little 
triangle  with  it  are  also  of  particular  interest.  Epsilon 
Lyrse,  which  is  the  northern  one  of  these  two  stars, 
may  be  used  as  a  test  of  keen  eyesight.  It  is  the 
finest  example  in  the  heavens  of  a  quadruple  star — that 
is,  "a  double-double  star."  A  keen  eye  can  just  separate 
this  star  into  two  without  a  telescope,  and  with  the  aid 
of  a  telescope,  each  of  the  two  splits  up  into  two  stars, 
making  four  stars  in  place  of  the  one  visible  to  the  aver- 
age eye.  Zeta,  the  other  of  the  two  stars  that  form  the 
little  triangle  with  Vega,  is  also  a  fine  double  star.  The 
star  that  lies  almost  in  a  straight  line  with  Epsilon  and 
Zeta  and  a  short  distance  to  the  south  of  them  is  a  very 
interesting  variable  star  known  as  Beta  Lyrae.  Its 
brightness  dianges  very  considerably  in  a  period  of 
twelve  days  and  twenty-two  hours.  This  change  of 


68        ASTRONOMY  FOR  YOUNG  FOLKS 

brightness  is  due  to  the  presence  of  a  companion  star. 
The  two  stars  are  in  mutual  revolution,  and  their  mo- 
tion is  viewed  at  such  an  angle  from  the  earth  that,  in 
each  revolution,  one  star  is  eclipsed  by  the  other,  pro- 
ducing a  variation  in  the  amount  of  light  that  reaches 
our  eyes.  By  comparing  this  star  from  day  to  day 
with  the  star  just  a  short  distance  to  the  southeast  of 
it,  which  does  not  vary  in  brightness,  we  can  observe 
for  ourselves  this  change  in  the  light  of  Beta  Lyrae. 
There  are  a  number  of  stars  in  the  heavens  that  vary 
in  brightness  in  the  same  manner  as  Beta  Lyrae,  and 
they  are  called  eclipsing-variable  stars. 

On  the  line  connecting  Beta  Lyrae  with  the  star 
southeast  of  it  and  one-third  of  the  distance  from  Beta 
to  this  star,  lies  the  noted  Ring  Nebula  in  Lyra,  which 
is  a  beautiful  object  even  in  a  small  telescope.  It  con- 
sists of  a  ring  of  luminous  gas  surrounding  a  central 
star.  The  star  shines  with  a  brilliant,  bluish-white 
light  and  is  visible  only  in  powerful  telescopes  though 
it  is  easily  photographed  since  it  gives  forth  rays  to 
which  the  photographic  plate  is  particularly  sensitive. 
In  small  telescopes  the  central  part  of  this  nebula  ap- 
pears dark  but  with  a  powerful  telescope  a  faint  light 
may  be  seen  even  in  the  central  portion  of  the  nebula. 
This  is  one  of  the  most  interesting  and  beautiful  tele- 
scopic objects  in  the  heavens. 

••vlt  is  in  the  general  direction  of  the  constellation  of 
Lyra  that  our  solar  system  is  speeding  at  the  rate  of 
more  than  a  million  miles  a  day.  This  point  toward 
Which  we  are  moving  at  such  tremendous  speed  lies  a 


AUGUST  69 

little  to  the  southwest  of  Vega,  on  the  border  between 
the  constellations  of  Lyra  and  Hercules,  and  is  spoken 
of  as  The  Apex  of  the  Sun's  Way. 

In  the  southern  sky  we  have  this  month  the  con- 


AUGUST — SAGITTARIUS 

stellation  of  Sagittarius,  The  Archer,  which  is  just  to 
the  east  of  Scorpio  and  a  considerable  distance  south  of 
Lyra.  It  can  be  recognized  by  its  peculiar  form,  which 
is  that  of  a  short-handled  milk  dipper,  with  the  bowl 
turned  toward  the  south  and  a  trail  of  bright  stars 
running  from  the  end  of  the  handle  toward  the  south- 
west. This  is  one  of  the  zodiacal  groups  which  contain 


70        ASTRONOMY  FOR  YOUNG  FOLKS 

no  first-magnitude  stars,  but  a  number  of  the  second 
and  third  magnitude.  It  is  crossed  by  the  Milky  Way, 
which  is  very  wonderful  in  its  structure  at  this  point. 
Some  astronomers  believe  that  here — among  the  star- 
clouds  and  mists  of  nebulous  light  which  are  intermingled 
with  dark  lanes  and  holes,  in  reality  dark  nebulae — lies 
the  center  of  the  vast  system  of  stars  and  nebulae  in 
which  our  entire  solar  system  is  but  the  merest  speck. 
Some  of  the  grandest  views  through  the  telescope  are 
also  to  be  obtained  in  this  beautiful  constellation  of 
Sagittarius,  which  is  so  far  south  that  it  is  seen  to 
better  advantage  in  the  tropics  than  in  the  mid-latitudes 
of  the  northern  hemispheres. 


8 

SEPTEMBER 

E  of  the  most  beautiful  constellations  of  the 
northern  hemisphere  is  Cygnus,  The  Swan,  which 
is  in  the  zenith  in  mid-latitudes  about  nine  o'clock  in 
the  evening  the  middle  of  September.  It  lies  directly  in 
the  path  of  the  Milky  Way  which  stretches  diagonally 
across  the  heavens  from  the  northeast  to  the  southwest 
at  this  time.  In  Cygnus,  the  Milky  Way  divides  into 
two  branches,  one  passing  through  Ophiuchus  and 
Serpens  to  Scorpio,  and  the  other  through  Sagitta  and 
Aquila  to  Sagittarius,  to  meet  again  in  the  southern 
constellation  of  Ara,  just  south  of  Scorpio  and  Sagit- 
tarius. On  clear,  dark  evenings,  when  there  is  no 
moonlight,  this  long,  dark  rift  in  the  Milky  Way  can 
be  seen  very  clearly.  In  Cygnus,  as  in  Ophiuchus, 
Scorpio,  and  Sagittarius  we  find  wonderful  star-clouds, 
consisting  of  numberless  stars  so  distant  from  us  and, 
therefore,  so  faint  that  they  do  not  appear  as  distinct 
points  of  light  except  in  the  greatest  telescopes.  It  is 
the  combined  light  from  these  numberless  stars  that 
cannot  be  seen  separately  that  produces  the  impression 
of  stars  massed  in  clouds  of  nebulous  light  and  gives 
to  this  girdle  of  the  heavens  its  name  of  the  Milky 
Way.  In  Cygnus,  as  in  a  number  of  other  constella- 

71 


72        ASTRONOMY  FOR  YOUNG  FOLKS 

tions  of  both  hemispheres,  the  Milky  Way  is  crossed 
by  dark  rifts  and  bars  and  is  very  complicated  in  its 
structure.  It  is  in  Cygnus,  also,  that  one  may  see  with 
the  aid  of  powerful  telescopes  the  vast,  irregular,  lumi- 


SEPTEMBER — CYGNUS 

nous  nebulae,  that  are  like  great  clouds  of  fiery  mist. 
These  nebulae  are  of  enormous  extent,  for  they  cover 
space  that  could  be  occupied  by  hundreds  of  stars. 

Cygnus  is  a  constellation  that  is  filled  with  the  wonders 
and  mysteries  of  space  and  that  abounds  in  beautiful 
objects  of  varied  kinds.  It  is  a  region  one  never  tires 
of  exploring  with  the  telescope.  The  principal  stars 
in  Cygnus  form  the  well-khown  Northern  Cross,  with 
the  beautiful,  white,  first-magnitude  star  Deneb,  or 
Arided,  as  it  is  sometimes  called,  at  the  top  of  the 
cross,  and  Albireo,  the  orange-and-blue  double  star 
at  the  foot.  Albireo,  among  all  the  pairs  of  contrast- 


SEPTEMBER  73 

ing  hues,  has  the  distinction  of  being  considered  the 
finest  double  star  in  the  heavens  for  small  telescopes. 
This  star  marks  the  head  of  The  Swan,  as  well  as  the 
foot  of  the  Northern  Cross,  and  Deneb  marks  the  tail 
of  The  Swan.  A  short  distance  to  the  southeast  of 
Deneb,  on  the  right  wing  of  The  Swan,  is  a  famous 
little  star,  61  Cygni,  'barely  visible  to  the  naked  eye  and 
forming  a  little  triangle  with  two  brighter  stars  to  the 
east.  This  star  has  the  distinction  of  being  the  first 
one  to  have  its  distance  from  the  solar  system  de- 
termined. The  famous  mathematician  and  astronomer 
Bessel  accomplished  this  difficult  feat  in  the  year  1838. 
Since  that  day,  the  distances  of  many  stars  have  been 
found  by  various  methods,  but  of  all  these  stars  only 
four  or  five  are  known  to  be  nearer  to  us  than  61 
Cygni.  Its  distance  is  about  eight  light-years,  so  its 
light  takes  about  eight  years  to  travel  the  distance  that 
separates  it  from  the  solar  system.  As  a  result,  we 
see  it  not  as  it  is  tonight,  but  as  it  was  at  the  time 
when  the  light  now  entering  our  eyes  first  started  on 
its  journey  eight  years  ago.  61  Cygni  is  also  a  double 
star,  and  the  combined  light  of  the  two  stars  gives 
forth  only  one-tenth  as  much  light  as  our  own  sun. 
Most  of  the  brilliant  first-magnitude  stars  give  forth 
many  times  as  much  light  as  the  sun;  but  among  the 
fainter  stars,  we  find  some  that  appear  faint  because 
they  are  very  distant,  and  some  that  are  faint  because 
they  are  dwarf  stars  and  have  little  light  to  give  forth. 
To  the  class  of  nearby,  feebly-shining  dwarf  stars  61 
Cygni  belongs.  Deneb,  on  the  other  hand,  is  one  of 


74        ASTRONOMY  FOR  YOUNG  FOLKS 

the  giant  stars,  and  is  at  an  immeasurably  great  dis- 
tance from  the  solar  system. 

Just  south  of  Cygnus  in  the  eastern  branch  of  the 
Milky  Way  lie  Sagitta,  The  Arrow,  and  Aquila,  The 
Eagle.  Not  far  to  the  northeast  of  Aquila  is  the  odd 
little  constellation  of  Delphinus,  The  Dolphin,  popularly 
referred  to  as  Job's  Coffin.  There  will  be  no  difficulty 
in  finding  this  small  star-group,  owing  to  its  peculiar 
diamond-shaped  configuration.  Its  five  principal  stars 
are  of  the  fourth  magnitude.  It  is  in  the  constellation 
of  Delphinus  that  the  most  distant  known  object  in  the 
heavens  is  located.  This  is  the  globular  star  cluster  known 
only  by  its  catalogue  number  of  N.G.C.  7006.  It  is 
estimated  to  be  at  a  distance  of  220,000  light-years  from 
the  earth. 

Sagitta,  The  Arrow,  lies  midway  between  Albireo 
and  the  brilliant  Altair  in  Aquila.  The  point  of  the 
arrow  is  indicated  by  the  star  that  is  farthest  east;  and 
the  feather,  by  the  two  faint  stars  to  the  west.  Like 
Delphinus,  this  constellation  is  very  small  and  contains 
no  objects  of  particular  interest. 

Altair  (Flying  Eagle)  is  the  brilliant  white  star  »of 
the  first  magnitude  in  Aquila  and  is  attended  by  two 
fainter  stars,  one  on  either  side,  at  nearly  equal  dis- 
tances from  it.  These  two  stars  serve  readily  to 
distinguish  this  star,  all  three  stars  being  nearly  in 
a  straight  line.  Altair  is  one  of  the  nearer  stars,  its 
distance  from  the  earth  being  about  sixteen  light-years. 
It  gives  forth  about  ten  times  as  much  light  as  the 
sun. 


SEPTEMBER  75 

We  cannot  leave  the  constellation  of  Aquila  without 
referring  to  the  wonderful  temporary  star  or  nova, 
known  as  Nova  Aquilse  No.  3  (because  it  was  the  third 
nova  to  appear  in  this  constellation),  which  appeared 


SEPTEMBER — DELPHINUS,  AQUILA  AND  SAGITTA 

in  the  position  indicated  on  the  chart  upon  the  eighth 
of  June,  1918.  A  few  days  previous  to  this  date,  there 
was  in  this  position  an  extremely  faint  star,  invisible 
to  the  naked  eye  and  in  small  telescopes.  This  fact 
became  known  from  later  examinations  of  old  photo- 
graphs of  this  region  that  had  been  taken  at  the 
Harvard  College  Observatory,  where  the  photographing 
of  the  heavens  is  carried  on  regularly  for  the  purpose 
of  having  a  record  of  celestial  changes  and  happenings. 
Clouds  prevented  the  obtaining  of  any  photographs  of 
this  part  of  the  heavens  on  the  four  nights  preceding  the 
eighth  of  June,  but  on  this  evening  there  shone  in  the 
place  of  the  faint  telescopic  star,  a  wonderful  tempo- 
rary star,  or  nova,  which  was  destined  on  the  next 


76        ASTRONOMY  FOR  YOUNG  FOLKS 

evening  to  outshine  all  stars  in  the  heavens,  with  the 
exception  of  the  brightest  of  all,  Sirius,  which  it 
closely  rivaled  in  brilliancy  at  the  height  of  its  outburst. 
Within  less  than  a  week's  time,  this  faint  star  in  the 
Milky  Way  for  some  mysterious  reason  increased  in 
brightness  many  thousandfold.  Such  outbursts  have 
been  recorded  before,  but  on  rare  occasions,  however. 
No  star  since  the  appearance  of  the  nova  known  as 
Kepler's  Star,  in  the  year  1604,  which  at  its  greatest 
brilliancy  rivaled  Jupiter,  shone  with  such  splendor  or 
attracted  so  much  attention  as  Nova  Aquilae.  In  the 
year  1901,  there  appeared  in  the  constellation  of 
Perseus  a  star  known  as  Nova  Persei  which  at  its 
brightest  surpassed  Vega,  but  its  splendor  was  not  as 
great  as  that  of  the  wonderful  nova  of  1918. 

It  speaks  well  for  the  zeal  and  interest  of  amateur 
astronomers,  as  well  as  for  their  acquaintance  with  the 
stars,  that  Nova  Persei  was  discovered  by  an  amateur 
astronomer,  Dr.  Anderson,  and  that  among  the  deluge 
of  telephone  calls  and  telegrams  received  at  the  Harvard 
College  Observatory  on  the  night  of  June  8th,  announc- 
ing independent  discoveries  of  the  "new  star,"  were 
many  from  non-professional  astronomers. 

Like  all  stars  of  this  class,  Nova  Aquilae  No.  3 
sank  rapidly  into  oblivion.  In  a  few  weeks  it  was  only 
a  third-magnitude  star;  a  few  weeks  more  and  it  was 
invisible  without  a  telescope.  Many  wonderful  and 
interesting  changes  have  been  recorded  in  the  appear- 
ance of  this  star,  however,  even  after  it  became  visible 
only  in  the  telescope.  Soon  after  its  outburst  it 


SEPTEMBER  77 

appeared  to  develop  a  nebulous  envelope,  as  have  other 
novas  before  it.  It  showed  in  addition  many  of  the 
peculiarities  of  the  nebulae,  though  the  central  star 
remained  visible  as  before  the  outburst. 

Astronomers  are  still  in  doubt  as  to  the  cause  of 
these  outbursts,  which  certainly  indicate  celestial  catas- 
trophies  of  some  form  on  a  gigantic  scale.  All  novas 
possess  one  characteristic  in  common — that  of  appearing 
exclusively  in  the  Milky  Way;  and  another  character- 
istic is  the  development  of  a  nebular  envelope  after  the 
outburst  of  greatest  brightness.  In  some  cases  tempo- 
rary stars  have  been  known  to  be  variable  in  bright- 
ness for  years  before  the  great  outburst.  Such  a  star 
was  Nova  Aquila,  for  the  examination  of  photographs 
of  this  region  taken  some  years  previous  showed  varia- 
tions in  its  brightness  for  a  period  of  thirty  years  at 
least. 

Up  to  the  beginning  of  this  century  only  about 
thirty  novas  had  been  discovered.  Since  that  date, 
thanks  to  the  vigilance  of  the  astronomers  of  today 
and  to  the  aid  of  photography,  more  have  been  dis- 
covered than  in  all  the  preceding  centuries.  These 
outbursts  of  new  stars  appear  to  be  not  so  rare  as  the 
earlier  astronomers  believed,  though  great  outbursts  as 
brilliant  as  that  of  Nova  Aquila  are  very  uncommon. 


XI 

OCTOBER 

THE   constellations   that   will  be    found    nearest   the 
meridian  in  early  October  evenings  are  the  circum- 
polar  constellations  Cepheus  and  Cassiopeia,  and  in  the 
southern  sky,  Capricornus  and  Aquarius. 

Cepheus,  The  King,  and  Cassiopeia,  his  Queen,  of 
whom  we  shall  have  more  to  say  later  in  connection 
with  the  constellations  of  Andromeda  and  Perseus,  sit 
facing  the  north  pole  of  the  heavens  opposite  Ursa 
Major,  The  Great  Bear,  familiar  to  us  under  the  name 
of  The  Big  Dipper.  The  foot  of  Cepheus  rests  upon 
the  tail  of  the  Little  Bear,  and  the  star  farthest  north 
in  the  diagram  is  in  the  left  knee.  The  head  is  marked 
by  a  small  triangle  of  faint  stars,  shown  in  the  dia- 
gram. One  of  these  three  faint  stars — the  one  farthest 
east — known  as  Delta  Cephei,  is  a  very  remarkable 
variable  star,  changing  periodically  in  brightness  every 
five  and  one-third  days.  Its  name  has  been  given  to  a 
large  class  of  variable  stars — the  Cepheid  variables — 
that  resemble  Delta  Cephei  in  being  giant  suns,  faint 
only  because  they  are  at  very  great  distances  from  the 
earth,  and  varying  in  brightness  with  the  greatest 
regularity  in  periods  that  range  from  a  few  hours  to 
several  weeks.  It  has  been  found  that  the  longer  the 
period  of  light  change  the  greater  is  the  star  in  size 

78 


OCTOBER  79 

and  brightness.  The  Cepheids  of  longest  period  are 
10,000  times  more  brilliant  than  our  own  sun.  Cepheus 
contains  no  very  bright  or  conspicuous  stars.  Alpha 
Cephei,  the  brightest  star  in  the  group,  marks  The 


.CEPMEUS 

/         *• 
/ 

CASSIO'PEIA  ^    V 


OCTOBER — CASSIOPEIA  AND  CEPHEUS 

King's  right  shoulder.  It  is  the  star  farthest  to  the 
west  in  the  diagram,  and  is  only  a  third-magnitude  star. 
Cassiopeia  is  a  constellation  with  which  every  one 
in  the  northern  hemisphere  should  be  familiar,  owing 
to  its  very  distinctive  W-shape  and  its  far  northern 
position,  which  brings  it  conspicuously  into  view 
throughout  the  clear  fall  and  winter  evenings.  Cassi- 


80        ASTRONOMY  FOR  YOUNG  FOLKS 

opeia  is  pictured  in  all  star  atlases  that  show  the  mytho- 
logical figures,  with  her  face  toward  the  north  pole. 
The  stars  in  the  W  outline  the  body.  Alpha,  the  star 
farthest  south  in  the  diagram  marks  the  breast  of  Cas- 
siopeia. Her  head  and  uplifted  hands  are  represented 
by  faint  stars  south  of  Alpha.  This  star  is  occasion- 
ally referred  to  by  its  Arabic  name  of  Schedir.  Beta, 
the  leader  of  all  the  stars  in  the  W  in  their 
daily  westward  motion,  is  also  known  by  an  Arabic 
name,  Caph. 

In  the  constellation  of  Cassiopeia  there  appeared  in 
the  year  1572  A.D.,  a  wonderful  temporary  star  which 
suddenly,  within  a  few  days'  time,  became  as  brilliant 
as  the  planet  Venus  and  was  clearly  visible  in  broad 
daylight.  This  star  is  often  referred  to  as  Tycho's 
Star,  because  it  was  observed,  and  its  position  very 
accurately  determined,  by  Tycho  Brahe,  one  of  the  most 
famous  of  the  old  astronomers.  This  star  remained 
visible  to  every  one  for  about  sixteen  months,  but 
it  finally  faded  completely  from  view,  and  it  is  believed 
that  a  faint,  nebulous  red  star,  visible  only  in  the 
telescope  and  close  to  the  position  recorded  by  Tycho, 
represents  the  smoldering  embers  of  the  star  that  once 
struck  terror  to  the  hearts  of  the  superstitious  and 
ignorant  among  all  the  nations  of  Europe,  who  took 
it  to  be  a  sign  that  the  end  of  the  world  was  at  hand. 

Both  Cassiopeia  and  Cepheus  lie  in  the  path  of  the 
Milky  Way,  which  reaches  its  farthest  northern  point 
in  Cassiopeia  and  passes  from  Cepheus  in  a  southerly 
direction  into  the  constellation  of  Cygnus. 


OCTOBER 


81 


Turning  now  to  southern  skies,  we  find  on  and  to  the 
west  of  the  meridian  at  this  time  the  rather  inconspicu- 
ous zodiacal  constellation  of  Capricornus,  The  Goat.  It 
contains  no  stars  of  great  brightness  and  is  chiefly  re- 


OCTOBER — AQUARIUS  AND  CAPRICORNUS 

markable  for  the  fact  that  it  contains  one  of  the  few 
double  stars  that  can  be  seen  without  the  aid  of  a 
telescope.  The  least  distance  in  the  heavens  that  the 
unaided  human  eye  can  separate  is  about  four  minutes 
of  arc.  The  star  Alpha  in  Capricornus  is  made  up 
of  two  stars  separated  by  a  distance  of  six  minutes 
of  arc,  so  that  any  one  can  readily  see  that  it  consists  of 


82    ASTRONOMY  FOR  YOUNG  FOLKS 

two  stars  very  close  together.  This  star,  Alpha,  will 
be  found  in  the  extreme  western  part  of  the  constella- 
tion, and  can  best  be  located  in  conjunction  with  the 
star  Beta,  which  is  slightly  brighter  and  lies  but  a  short 
distance  almost  due  south  of  Alpha,  the  two  stars 
standing  somewhat  alone  in  this  part  of  the  heavens. 

To  the  north  and  east  of  Capricornus  we  find  Aquar- 
ius, which  is  also  a  zodiacal  constellation.  Aquarius 
is  the  Water-Bearer,  and  the  water  jar  which  he 
carries  is  represented  by  a  small,  but  distinct,  Y  of 
stars  from  which  flows  a  stream  of  faint  stars  toward 
the  southeast  and  south.  Aquarius,  like  Capricornus, 
is  a  rather  uninteresting  constellation,  as  it  is  made  up 
of  inconspicuous  third-  and  fourth-magnitude  stars. 
The  entire  region  covered  by  these  two  groups  of  stars 
is  remarkably  barren,  since  it  contains  not  a  single 
first-  or  even  second-magnitude  star  and  little  to  attract 
the  observer's  eye. 

To  relieve  the  barrenness  of  this  region,  there  ap- 
pears just  to  the  south  of  Aquarius  and  southeast  of 
Capricornus,  sparkling  low  in  the  southern  sky  on 
crisp  October  evenings,  the  beautiful  first-magnitude 
star  Fomalhaut  in  the  small  southern  constellation  of 
Piscis  Australis,  The  Southern  Fish.  This  star  is 
the  farthest  south  of  all  the  brilliant  first-magnitude 
stars  that  can  be  seen  from  the  middle  latitudes  of  the 
Northern  Hemisphere.  The  constellation  in  which  it 
lies  is  so  close  to  the  southern  horizon  in  our  latitudes 
that  it  cannot  be  seen  to  any  advantage,  and  it  is  at 
best  very  inconspicuous,  containing  no  other  objects 


OCTOBER  83 

of  interest.  Fomalhaut  cannot  be  mistaken  for  any 
other  star  visible  at  this  time  of  year  in  the  evening, 
since  it  stands  in  such  a  solitary  position  far  to  the 
south.  At  the  time  of  which  we  are  writing  it  will  be 
found  a  few  degrees  east  of  the  meridian. 


XII 
NOVEMBER 

DIRECTLY  south  of  Cassiopeia  and  Cepheus,  the 
circumpolar  constellations  with  which  we  became 
acquainted  last  month,  and  almost  overhead  in  our  lati- 
tudes in  the  early  evening  hours  of  November,  lie 
Pegasus,  The  Winged  Horse,  and  Andromeda,  The 
Woman  Chained. 

According  to  the  legend,  Cepheus  was  king  of 
Ethiopia,  and  Cassiopeia  was  the  beautiful,  but  vain, 
queen  who  dared  to  compare  herself  in  beauty  with 
the  sea-nymphs.  This  so  enraged  the  nymphs  that, 
as  a  punishment  for  her  presumption,  they  decided 
to  send  a  terrible  sea-monster  to  ravage  the  coast  of  the 
kingdom.  The  king  and  queen,  upon  consulting  the 
oracle,  found  that  the  only  way  to  avert  this  calamity 
would  be  to  chain  their  daughter  Andromeda  to  the 
rocks  and  permit  the  monster  to  devour  her. 

As  the  story  goes,  the  valiant  hero,  Perseus,  chanced 
to  be  riding  through  the  air  on  his  winged  horse  and 
saw,  far  beneath  him  the  beautiful  maiden  chained  to 
the  rocks  and  the  frightful  monster  approaching  to 
devour  her.  He  immediately  went  to  the  rescue,  and, 
after  a  terrible  struggle  with  the  monster,  succeeded  in 
overpowering  him  and  thus  saved  the  maiden  from  a 

84 


NOVEMBER 


85 


dreadful  fate.  Perseus  and  the  fair  Andromeda  were 
married  shortly  afterward,  and  at  the  end  of  a  happy 
life  the  pair  were  transferred  to  the  heavens.  Cassi- 


NOVEMBER — ANDROMEDA  AND  PEGASUS 


opeia,  the  vain  queen,  was  ordered  to  be  bound  to  a 
chair  and,  with  the  king  Cepheus  at  her  side,  to  be 
swung  continually  around  the  north  pole  of  the  heavens 
that  she  might  be  taught  a  lesson  in  humility. 

The  constellation  Cetus,  representing  the  sea-monster, 
will  be  found  to  the  southeast  and  south  of  Pisces, 
The  Fishes,  which  lie  south  of  Andromeda  and  Pegasus. 

The  Great  Square  in  Pegasus  is  the  most  conspicuous 


86        ASTRONOMY  FOR  YOUNG  FOLKS 

configuration  of  stars  to  be  seen  in  the  heavens  in 
autumn  evenings.  The  star  that  marks  the  north- 
eastern corner  of  The  Great  Square  belongs  to  the 
constellation  of  Andromeda  and  marks  the  head  of  the 
maiden,  who  is  resting  upon  the  shoulders  of  Pegasus, 
The  Winged  Horse.  The  three  bright  stars  nearly  in 
a  straight  line  outline  the  maiden's  body,  Alpha,  or 
Alpheratz,  as  it  is  called,  being  the  star  in  the  head, 
Beta  or  Mirach  in  the  waist,  and  Gamma  or  Almach 
in  the  left  foot.  The  last-named  star,  which  is  farthest 
to  the  northeast  in  the  diagram,  was,  in  the  opinion 
of  the  noted  astronomer  Herschel,  the  finest  double  star 
in  the  heavens.  The  two  stars  into  which  the  telescope 
splits  it  are  of  the  beautifully  contrasted  shades  of 
orange  and  sea  green. 

A  second  most  interesting  object  in  Andromeda 
and  one  of  the  finest  in  the  entire  heavens  is  The  Great 
Andromeda  Nebula,  which  is  faintly  visible  without 
the  aid  of  a  telescope  as  a  hazy  patch  of  light.  It  is 
believed  that  in  reality  this  nebula  is  a  great  universe 
composed  of  many  thousands  of  stars  so  distant  that 
no  telescope  can  show  the  individual  members  and  that 
the  light  from  it  takes  many  thousands  of  years  to 
span  the  abyss  that  separates  it  from  the  solar  system. 
Some  magnificent  photographs  of  The  Great  Andro- 
meda Nebula  have  been  taken  with  powerful  tele- 
scopes. It  is  through  the  use  of  photography  that 
the  nebulae  can  best  be  studied,  for  a  photographic 
plate  after  long  exposure,  reveals  a  wonderful  detail 
in  the  structure  of  these  objects  that  the  human  eye 


NOVEMBER  87^ 

fails  to  see.  On  a  clear,  dark  evening  one  may  find 
The  Great  Andromeda  Nebula  by  the  aid  of  two  faint 
stars  with  which  it  makes  a  small  triangle,  as  shown 
in  the  chart.  This  nebula  is  the  only  one  of  the  spiral 
nebula  that  can  be  seen  in  these  latitudes  without  the 
aid  of  a  telescope,  though  there  are  several  spiral 
nebulae  in  the  southern  heavens  that  can  be  thus  seen. 

Lying  to  the  northwest  of  The  Great  Square  in 
Pegasus  are  a  number  of  faint  stars  that  outline  the 
shoulders  and  head  of  the  winged  steed,  while  the 
stars  to  the  southwest  of  the  square  outline  his  fore- 
legs. The  creature  is  represented  without  hind  quar- 
ters in  all  star  atlases.  The  space  within  The  Great 
Square  contains  no  bright  stars,  and  as  a  result,  the 
outline  of  the  square  stands  out  with  great  distinctness. 
There  are,  in  fact,  no  stars  of  the  first  magnitude  in 
either  Pegasus  or  Andromeda,  though  there  are  a 
number  of  the  second  and  third  magnitude  which  very 
clearly  show  the  distinctive  forms  of  these  two  star- 
groups. 

Pisces,  The  Fishes,  the  constellation  just  south  of 
Andromeda  and  Pegasus,  is  the  first  of  the  twelve 
zodiacal  constellations.  It  consists  of  the  southern 
fish,  lying  in  an  east-to-west  direction,  and  the  northern 
fish,  lying  nearly  north  and  south,  the  two  touching  at 
the  southeastern  extremity  of  the  constellation. 

There  is  in  Pisces  not  a  single  bright  star,  and  its 
only  point  of  interest  is  to  be  found  in  the  fact  that 
it  contains  the  point,  marked  by  the  cross  and  letter  V 
in  the  diagram,  that  is  known  variously  as  "the  vernal 


88        ASTRONOMY  FOR  YOUNG  FOLKS 

equinox,"  "the  equinoctial  point"  and  "The  First  Point 
in  Aries."  This  is  a  very  important  point  of  reference 
in  the  heavens,  just  as  the  meridian  of  Greenwhich  is 
for  the  earth,  and  it  marks  the  point  where  the  sun 


NOVEMBER — PISCES 


crosses  the  equator  going  north  in  the  spring.  Owing 
to  the  Precession  of  the  Equinoxes,  as  it  is  called, 
this  point  is  gradually  shifting  its  position  westward 
through  the  zodiacal  constellations  at  a  rate  that  will 
carry  it  completely  around  the  heavens  through  the 
twelve  zodiacal  groups  in  a  period  of  25,800  years. 


NOVEMBER  89 

Since  the  beginning  of  the  Christian  era,  this  point  has 
backed  from  the  constellation  of  Aries,  which  lies 
just  east  of  Pisces,  into  Pisces,  though  it  still  retains 
its  name  of  "The  First  Point  in  Aries.'* 


XIII 

DECEMBER 

THE  eastern  half  of  the  sky  on  early  December 
evenings  is  adorned  with  some  of  the  finest  star- 
groups  in  the  heavens;  but  as  we  are  considering  for 
each  month  only  the  constellations  that  lie  on  or  near 
the  meridian  in  the  early  evening  hours,  we  must  turn 
our  eyes  for  the  present  from  the  sparkling  brilliants  in 
the  east  to  the  stars  in  the  less  conspicuous  groups  of 
Aries,  The  Ram,  and  Cetus,  The  Whale.  We  will 
also  become  acquainted  this  month  with  the  beautiful 
and  interesting  constellation  of  Perseus,  the  hero  of 
mythical  fame  to  whom  we  referred  last  month  in  con- 
nection with  the  legend  of  Cepheus  and  Cassiopeia. 
Cetus,  you  will  recall,  represents  the  sea-monster  sent 
to  devour  Andromeda,  the  daughter  of  Cepheus  and 
Cassiopeia.  We  have  included  the  constellation  of 
Andromeda  in  our  diagram  for  this  month,  since  it 
is  so  closely  associated  in  legend  with  the  constellations 
of  Perseus  and  Cetus,  though  we  also  showed  it  last 
month. 

The  brightest  star  in  Perseus,  known  as  Alpha 
Persei,  is  at  the  center  of  a  curved  line  of  stars  that 
is  concave  or  hollow  toward  the  northeast.  This  line 
of  stars  is  called  the  Segment  of  Perseus,  and  it  lies 

90 


DECEMBER  91 

along  the  path  of  the  Milky  Way,  which  passes  from 
this  point  in  a  northwesterly  direction  into  Cassiopeia. 
According  to  the  legend,  Perseus,  in  his  great  haste 
to  rescue  the  maiden  from  Cetus,  the  monster,  stirred 
up  a  great  dust,  which  is  represented  by  the  numberless 
faint  stars  in  the  Milky  Way  at  this  point.  The  star 
Alpha  is  in  the  midst  of  one  of  the  finest  regions  of 
the  heavens  for  the  possessor  of  a  good  field-glass 
or  small  telescope. 

A  short  distance  to  the  southwest  of  Alpha  is  one  of 
the  most  interesting  objects  in  the  heavens.  To  the 
ancients,  it  represented  the  baleful,  winking  demon-eye 
in  the  head  of  the  snaky-locked  Gorgon,  Medusa,  whom 
Perseus  vanquished  in  one  of  his  earliest  exploits  and 
whose  head  he  carried  in  his  hand  at  the  time  of  the 
rescue  of  Andromeda.  To  the  astronomers,  however, 
Algol  is  known  as  Beta  Persei,  a  star  that  has  been 
found  to  consist  of  two  stars  revolving  about  each 
other  and  separated  by  a  distance  not  much  greater  than 
their  own  diameters.  One  of  the  stars  is  so  faint  that 
we  speak  of  it  as  a  dark  star,  though  it  does  emit  a 
faint  light.  Once  in  every  revolution  the  faint  star 
passes  directly  between  us  and  the  bright  star  and 
partly  eclipses  it,  shutting  off  five-sixths  of  its  light. 
This  happens  with  great  regularity  once  in  a  little  less 
than  three  days.  It  is  for  this  reason  that  Algol 
varies  in  brightness  in  this  period.  There  are  a  number 
of  stars  that  vary  in  brightness  in  a  similar  manner. 
Their  periods  of  light-change  are  all  very  short,  and 
the  astronomers  call  them  eclipsing  variables.  At  its 


92        ASTRONOMY  FOR  YOUNG  FOLKS 

brightest,  Algol  is  slightly  brighter  than  the  star  nearest 
to  it  in  Andromeda,  which  is  an  excellent  star  with 
which  to  compare  it. 

Perseus   is  another   one  of  the   constellations   lying 


PERSEUS;' 


DECEMBER — PERSEUS,  ARIES  AND  CETUS 

in  the  Milky  Way  in  which  temporary  stars  or  novas 
have  suddenly  flashed  forth.  At  the  point  indicated  by 
a  cross  in  the  diagram,  Dr.  Anderson,  an  amateur 
astronomer  of  Scotland,  found  on  February  21,  1901, 
a  new  star  as  brilliant  as  the  pole-star.  On  the  follow- 
ing day  it  became  brighter  than  a  star  of  the  first 
magnitude.  A  day  later  it  had  lost  a  third  of  its 


DECEMBER  93 

light,  and  in  a  few  weeks  it  was  invisible  without  the 
aid  of  a  telescope.  In  a  year  it  was  invisible  in  all 
except  the  most  powerful  telescopes.  With  such  tele- 
scopes, it  may  still  be  seen  as  a  very  faint  nebu- 
lous light. 

Triangulum  and  Aries  are  two  rather  inconspicuous 
constellations  that  lie  on,  or  close  to,  the  meridian  at 
this  time.  There  is  nothing  remarkable  about  either 
of  these  groups,  except  that  Aries  is  one  of  the 
twelve  zodiacal  constellations.  Some  centuries  ago, 
the  sun  was  to  be  found  in  Aries  at  the  beginning  of 
spring  and  the  position  it  occupies  in  the  sky  at  that 
time  was  called  the  First  Point  in  Aries.  As  this 
point  is  slowly  shifting  westward,  as  we  have  explained 
elsewhere,  the  sun  is  now  to  be  found  in  Pisces,  instead 
of  Aries  at  the  beginning  of  spring  and  does  not  enter 
Aries  until  a  month  later.  Pisces  was  one  of  the  con- 
stellations for  November  and  we  showed  in  that  con- 
stellation the  present  position  of  the  sun  at  the  begin- 
ning of  spring. 

Two  stars  in  Aries — Alpha  and  Beta — are  fairly 
bright,  Alpha  being*  fully  as  bright  as  the  brightest 
star  in  Andromeda.  Beta  lies  a  short  distance  to  the 
southwest  of  Alpha,  and  a  little  to  the  southwest  of 
Beta  is  Gamma,  the  three  stars  forming  a  short  curved 
line  of  stars  that  distinguishes  this  constellation  from 
other  groups.  The  remaining  stars  in  Aries  are 
all  faint. 

Just  south  of  Aries  lies  the  head  of  Cetus,  The 
Whale.  This  is  an  enormous  constellation  that  ex- 


94        ASTRONOMY  FOR  YOUNG  FOLKS 

tends  far  to  the  southwest,  below  a  part  of  Pisces, 
which  runs  in  between  Andromeda  and  Cetus.  Its 
brightest  star,  Beta,  Diphda,  or  Deneb  Kaitos,  as  it 
is  severally  called,  stands  quite  alone  not  far  above  the 
southwestern  horizon.  It  is  almost  due  south  of  Alpha 
Andromedae,  the  star  in  Andromeda  farthest  to  the 
west,  which  it  exactly  equals  in  brightness.  The  head 
of  Cetus  is  marked  by  a  five-sided  figure  composed  of 
stars  that  are  all  faint  with  the  single  exception  of 
Alpha,  which  is  fairly  bright,  though  inferior  to  Beta 
or  Diphda. 

Cetus,  though  made  up  chiefly  of  faint  stars,  and 
on  the  whole  uninteresting,  contains  one  of  the  most 
peculiar  objects  in  the  heavens,  the  star  known  as 
Omicron  Ceti  or  Mira  (The  Wonderful).  This  star 
suddenly  rises  from  invisibility  nearly  to  the  bright- 
ness of  a  first-magnitude  star  for  a  short  period  once 
every  eleven  months.  Mira  was  the  first  known  vari- 
able star.  Its  remarkable  periodic  change  in  bright- 
ness was  discovered  by  Fabricius  in  the  year  1596, 
so  its  peculiar  behavior  has  been  under  observation  for 
three  hundred  and  twenty-five  years.  It  is  called  a 
long-period  variable  star,  because  its  variations  of  light 
take  place  in  a  period  of  months  instead  of  a  few  hours 
or  days,  as  is  the  case  with  stars  such  as  Algol.  Mira 
is  not  only  a  wonderful  star,  it  is  a  mysterious  star  as 
well,  for  the  cause  of  its  light-changes  are  not  known, 
as  in  the  case  of  Algol  where  the  loss  of  light  is 
produced  by  a  dark  star  passing  in  front  of  a 
brighter  star.  Mira  is  a  deep-red  star,  as  are  all  long- 


DECEMBER  95 

period  variable  stars  that  change  irregularly  in  bright- 
ness. It  is  visible  without  a  telescope  for  only  one 
month  or  six  weeks  out  of  the  eleven  months.  During 
the  remainder  of  this  time,  it  sometimes  loses  so  much 
of  its  light  that  it  cannot  be  found  with  telescopes  of 
considerable  size.  Its  periods  of  light-change  are 
quite  variable  as  is  also  the  amount  of  light  it  gains 
at  different  appearances. 

It  is  believed  that  the  cause  of  the  light-changes  of 
Mira  is  to  be  found  within  the  star  itself.  It  has  been 
thought  that  dense  clouds  of  vapors  may  surround 
these  comparatively  cool,  red  stars  and  that  the  im- 
prisoned heat  finally  bursts  through  these  vapors  and 
we  see  for  a  short  time  the  glowing  gases  below;  then 
the  vapors  once  more  collect  for  a  long  period,  to  be 
followed  by  another  sudden  outburst  of  heat  and  light. 

It  is  interesting  to  remember  in  this  connection  that 
our  own  sun  has  been  found  to  be  slightly  variable 
in  the  amount  of  light  and  heat  that  it  gives  forth  at 
different  times,  and  the  cause  of  its  changes  in  light 
and  heat  are  believed  to  lie  within  the  sun  itself. 


XIV 

STARS  OF  THE  SOUTHERN  HEMISPHERE 

As  one  travels  southward  from  the  mid-latitudes  of 
the  northern  hemisphere  into  the  tropics  our  familiar 
circumpolar  constellations  sink  lower  and  lower  in  the 
northern  heavens  and  strange  and  unfamiliar  star- 
groups  rise  gradually  above  the  southern  horizon.  If 
we  make  our  southward  journey  in  the  winter  months 
the  first  of  the  southern  constellations  to  come  fully 
into  view  is  the  small  star-group  just  south  of  Lepus 
known  as  Columba  (The  Dove),  whose  brightest 
star  Phact  is  of  the  second  magnitude.  A  line  drawn 
from  Procyon  to  Sirius  and  extended  as  far  again 
brings  us  to  this  star  and  a  line  from  Betelgeuze  to 
Sirius  extended  an  equal  distance  brings  us  to  Zeta 
Argus  which  is  equal  to  Phact  in  brightness.  The  two 
lines  intersecting  at  Sirius  make  the  "Egyptian  X" 
as  it  is  called. 

Magnificent,  blue-white  Canopus,  the  most  brilliant 
star  in  the  heavens  next  to  Sirius,  a  veritable  diamond 
sparkling  low  in  the  southern  sky,  now  commands  our 
unqualified  admiration.  Canopus  lies  about  35°  south 
of  Sirius  and  is  invisible  north  of  the  37th  parallel 
of  latitude.  At  nine  o'clock  in  the  evening  of  February 
6th  it  can  be  seen  just  above  the  southern  horizon  in 

96 


STARS  OF  THE  SOUTHERN  HEMISPHERE      97 

that  latitude  and  is  then  a  conspicuous  object  in  Georgia, 
Florida  and  the  Gulf  States. 

"The  star  of   Egypt  whose  proud   light 
Never  hath  beamed  on  those  who  rest 
In  the  White  Islands  of  the  West/' 

writes  Moore  of  Canopus  in  "Lalla  Rookh." 

Along  the  Nile  Canopus  was  an  object  of  worship 
as  the  god  of  waters.  At  the  time  of  their  erection, 
6400  B.C.,  a  number  of  temples  in  Upper  Egypt  were 
oriented  so  as  to  show  Canopus  at  sunrise  at  the 
autumnal  equinox,  and  other  temples  erected  many 
centuries  later  were  oriented  in  a  similar  manner.  In 
China,  as  late  as  100  B.C.,  and  in  India  also  Canopus 
was  an  object  of  worship. 

The  astronomer  tells  us  that  Canopus  is  immeasur- 
ably distant  from  the  earth.  It  has  been  estimated  to  be 
forty  thousand  times  more  luminous  than  our  sun. 
Canopus  is  located  in  the  constellation  of  Argo 
Navis  which  is  the  largest  and  most  conspicuous  con- 
stellation in  the  heavens.  In  addition  to  Canopus  it 
contains  a  number  of  second-  and  third-magnitude 
stars  and  is  subdivided  for  convenience  into  Puppis, 
The  Prow;  Carina,  The  Keel;  and  Vela,  The  Sails. 
Huge  as  it  is,  Argo  Navis  represents  only  half  of  a 
ship  for  the  stern  is  lacking.  According  to  the  legend 
this  ship  was  built  by  Argos,  aided  by  Pallas  Athene, 
for  Jason,  the  leader  of  the  expedition  of  the  fifty 
Argonauts  who  sailed  from  Greece  to  Colchis  in  search 
of  the  golden  fleece.  Pallas  Athene  placed  in  the  bow 


98 


ASTRONOMY  FOR  YOUNG  FOLKS 


of  the  ship  a  piece  of  timber  from  the  speaking  oak 
of  Dodona  to  guide  the  crew  and  warn  them  of  dangers 
and  after  the  voyage  the  ship  was  supposed  to  have 
been  placed  in  the  heavens. 


SOUTHERN  CONSTELLATIONS — i.  IN  FEBRUARY 

In  Argo  Navis  is  one  of  the  finest  telescopic  objects 
in  the  heavens,  the  Keyhole  Nebula,  as  it  is  usually 
called,  from  a  peculiar-shaped  dark  patch  in  its  brightest 
part.  On  the  border  of  this  nebula  is  the  deep-red 
Wonder  Star  of  the  southern  hemisphere,  Eta  Argus, 
which  varies  suddenly  and  unexpectedly  in  brightness 


STARS  OF  THE  SOUTHERN  HEMISPHERE      99 

between  the  seventh  and  first  magnitudes.  In  1843  it 
burst  forth  with  a  splendor  rivaling  Sirius  and  main- 
tained this  brilliancy  for  nearly  ten  years  and  then 
slowly  waned  in  brilliancy  until  it  disappeared  to  the 
unaided  eye  in  1886.  The  surrounding  nebula  also 
seems  to  share  in  its  peculiar  fluctuations  of  bright- 
ness. Eta  Argus  is  now  a  star  of  the  seventh  magni- 
tude and  since  it  is  still  varying  fitfully  in  brightness 
it  is  believed  that  the  history  of  its  light-changes  is 
not  complete. 

Among  the  constellations  of  the  southern  heavens 
near  the  meridian  in  February  we  see  in  addition  to 
Argo  Navis  the  constellations  of  Dorado,  The  Gold- 
fish; Hydrus,  The  Serpent,  and  Tucana,  The  Toucan. 
Though  insignificant  in  appearance  Dorado  contains 
what  was  described  by  Sir  John  Herschel  as  one  of 
the  most  extraordinary  objects  in  the  heavens,  a 
worthy  rival  of  The  Great  Orion  Nebula  and  in  some 
respects  very  similar  to  it,  The  Great  Looped  Nebula, 
"the  center  of  a  great  spiral."  In  Dorado  also  is  lo- 
cated The  Greater  Magellanic  Cloud  which  looks  like 
a  detached  portion  of  the  Milky  Way  though  it  is 
far  removed  from  it.  To  the  naked  eye  it  resembles  a 
small  white  cloud  about  4°  in  extent.  In  the  telescope 
it  bears  a  close  resemblance  to  a  typical  portion  of 
the  Milky  Way.  A  similar  formation  known  as  The 
Lesser  Magellanic  Cloud  is  located  in  Hydrus.  It  has 
been  estimated  that  the  distance  of  The  Lesser  Cloud 
is  80,000  light-years  and  that  it  is  receding  from  us. 

In  Tucana  is  located  one  of  the  finest  globular  star 


100      ASTRONOMY  FOR  YOUNG  FOLKS 

clusters  in  the  heavens,  known  as  47  Tucanae.  This 
cluster  and  Omega  Centauri,  a  globular  star  cluster  in 
Centaurus,  are  the  two  nearest  of  all  the  globular  clus- 
ters. They  are  distant  from  the  earth  about  22,000 
light-years  and  it  is  known  that  the  combined  light  of 
the  thousands  of  stars  of  which  each  cluster  is  composed 
is  about  one  million  times  that  of  our  own  sun. 

In  the  western  sky  in  the  southern  hemisphere  in 
February  may  be  seen  the  brilliant,  white,  first-magni- 
tude star  Achernar  in  the  river  Eridanus,  the  long, 
winding  constellation  that,  we  recall,  starts  near  the 
brilliant  Rigel  in  Orion  and  disappears  from  the  view 
of  northern  observers  below  the  southern  horizon, 
extending  its  course  far  into  the  southern  hemisphere. 
Achernar  means  "The  End  of  the  River"  and  this  is 
nearly  its  position  in  the  constellation. 

Though  Argo  Navis  is  the  largest  and  most  im- 
portant constellation  of  the  southern  hemisphere,  Crux, 
The  Southern  Cross,  far-famed  in  story  and  legend 
as  well  as  for  its  historical  associations,  is  beyond  a 
doubt  the  most  popular. 

The  best  time  to  view  the  Southern  Cross  is  in 
June  or  July  when  it  is  near  the  meridian.  It  is  not 
seen  to  advantage  in  the  months  of  January  or  Feb- 
ruary. It  then  lies  on  its  side  and  close  to  the  horizon 
and  therefore  is  dimmed  by  atmospheric  haze  so  that 
it  almost  invariably  is  a  disappointing  object  to  the 
tourist  from  the  north  who  usually  views  it  for  the 
first  time  in  one  of  these  months.  The  Cross  is 
viewed  to  advantage  in  the  latitude  of  Rio  or  Val- 


STARS  OF  THE  SOUTHERN  HEMISPHERE     101 

paraiso  and  it  is  best  seen  from  the  Straits  where  it 
rides  high  overhead.  It  is  not  seen  to  advantage  from 
the  latitudes  of  Cuba  or  Jamaica.  It  is  small,  only 
6°  in  extent  from  north  to  south  and  less  in  width 
and  it  lies  in  the  most  brilliant  portion  of  the  Milky 
Way  which  is  here  a  narrow  stream  only  three  or 
four  degrees  wide.  Directly  below  the  Cross  is  the 
noted  Coal  Sack,  apparently  a  yawning  chasm  in  the 
midst  of  its  brilliant  surroundings  though  probably  in 
reality  a  dark  nebula.  Viewed  with  the  telescopes  a 
number  of  stars  are  to  be  seen  projected  on  this  dark 
background. 

The  Southern  Cross  is  to  the  inhabitants  of  the 
southern  hemisphere  what  the  Big  Dipper  is  to  those 
who  dwell  in  the  northern  hemisphere — an  infallible 
timepiece.  The  upright  of  the  Cross  points  toward 
the  south  pole  of  the  heavens  which  lies  in  a  region 
where  there  is  a  singular  dearth  of  bright  stars,  the 
nearest  star  to  the  south  pole  being  a  faint  fifth-magni- 
tude star  called  Sigma  Octantis.  When  seen  in  the 
southeast  or  southwest  the  Cross  lies  on  its  side,  but 
when  passing  the  meridian  it  stands  nearly  upright. 
Humboldt,  referring  to  this  fact,  says: 

"How  often  have  we  heard  our  guides  exclaim  in 
the  savannahs  of  Venezuela  and  in  the  desert  extending 
from  Lima  to  Truxillo,  'Midnight  is  past,  the  Cross 
begins  to  bend.' ' 

By  the  explorers  of  the  sixteenth  century  the  Cross 
was  taken  as  a  sign  of  heaven's  approval  of  their 


102      ASTRONOMY  FOR  YOUNG  FOLKS 

attempt  to  establish  the  Christian  religion  in  the  wilds 
of  the  New  World.  This  thought  is  beautifully  ex- 
pressed in  Mrs.  Hemans'  lines  in  "The  Cross  of  the 
South." 

"But  to  thee,  as  thy  lode-stars  resplendently  burn 
In  their  clear  depths  of  blue,  with  devotion  I  turn 
Bright  Cross  of  the   South!  and  beholding  thee  shine, 
Scarce  regret  the  loved  land  of  the  olive  and  vine. 
Thou  recallest  the  ages  when   first  o'er  the  main 
My  fathers  unfolded  the  ensign  of  Spain, 
And  planted  their   faith  in  the  regions  that  see 
Its    imperishing    symbol    ever   blazoned    in    thee." 

Alpha  Crucis,  the  brightest  star  in  Crux,  is  at  the 
foot  of  the  Cross.  It  consists  in  reality  of  two  second- 
magnitude  stars  forming  a  beautiful  double  while  a 
third  fifth-magnitude  star  one  and  one-half  minutes  of 
arc  distant  makes  with  this  pair  a  combination  similar 
to  our  Mizar  and  Alcor  of  the  Big  Dipper  though 
the  separation  is  not  great  enough  to  be  visible  to  the 
naked  eye.  The  second-magnitude  star  at  the  head 
of  the  Cross  is  a  deep  orange  in  color  and  the  two 
stars  that  mark  the  ends  of  the  cross-arm  are  white 
third-magnitude  stars. 

One  of  the  finest  constellations  of  the  southern 
hemisphere  is  Centaurus,  The  Centaur,  which  sur- 
rounds Crux  on  the  north  and  is  more  than  60°  in 
length.  Its  center  lies  about  50°  south  of  Spica  in 
Virgo  and  below  the  tail  of  Hydra.  Alpha  Centauri, 
its  brightest  star  and  the  nearest  star  to  the  solar  sys- 
tem, four  and  one-third  light-years  away,  is  a  golden- 


STARS  OF  THE  SOUTHERN  HEMISPHERE     103 

yellow  double  star  that  forms  with  the  star  Beta  Cen- 
tauri  on  the  west  a  configuration  similar  to  that  of 
Castor  and  Pollux  in  Gemini,  only  one  that  is  far  more 
striking  because  of  the  superior  brilliancy  of  the  stars. 


CAR  I N  A 


AUSTRAL^ 


ARA 


. 
CRUX  ^cruets      r 


CENTAURUS 


LUPU  S 


SOUTHERN  CONSTELLATIONS— 2.  IN  JULY 

Alpha  Centauri  lies  in  the  Milky  Way  and  transits  the 
meridian  at  the  same  time  with  Arcturus  though  it 
cannot  be  seen  north  of  the  29th  parallel.  Alpha 
Centauri,  like  Canopus,  was  an  object  of  worship  in 
Egypt  and  a  number  of  temples  in  northern  Egypt 
were  oriented  to  its  emergence  from  the  sun's  rays  in 


104   ASTRONOMY  FOR  YOUNG  FOLKS 

the  morning  at  the  autumnal  equinox,  between  3000 
and  2575  B.C. 

North  of  Centaurus  is  the  constellation  Lupus,  The 
Wolf,  which  is  also  crossed  by  the  Milky  Way.  Ac- 
cording to  one  myth  Lupus  is  held  in  the  right  hand 
of  the  Centaur  as  an  offering  upon  the  altar  which 
is  represented  by  the  constellation  of  Ara  next  to 
Centaurus  on  the  east.  Ara  also  is  crossed  by  the 
Milky  Way.  Neither  Lupus  nor  Ara  contain  any 
objects  that  are  worthy  of  special  attention. 

Triangulum  Australe,  The  Southern  Triangle,  a 
little  to  the  southeast  of  Alpha  Centauri,  is  far  more 
conspicuous  than  the  Triangulum  of  the  northern 
hemisphere. 

The  accompanying  charts  give  two  views  of  these 
principal  southern  constellations  that  lie  within  40° 
of  the  south  pole  of  the  heavens  and  that  are  below 
the  horizon  in  40°  north  latitude.  The  first  of  these 
charts  shows  the  constellations  that  are  nearest  the 
meridian  in  the  early  evening  hours  in  February. 
Canopus  in  Argo  Navis  and  the  Greater  Magellanic 
Cloud  then  lie  close  to  the  meridian.  Argo  Navis 
with  its  subdivisions  Puppis,  Vela  and  Carina  are 
found  east  of  the  meridian  lying  directly  in  the  path 
of  the  Milky  Way,  which  stretches  diagonally  across 
the  sky  from  the  northwest  to  the  southeast.  Far 
over  in  the  southeast  appears  Crux,  the  Southern 
Cross,  also  directly  in  the  path  of  the  Milky  Way. 
In  the  western  heavens  may  be  seen  the  Lesser  Magel- 


STARS  OF  THE  SOUTHERN  HEMISPHERE     105 

lanic  Cloud  in  Hydrus,  brilliant  Achernar  in  Eridanus 
and  the  inconspicuous  star-group  of  Tucana. 

In  the  early  evening  hours  of  July  we  find  as  shown 
on  the  second  chart,  Alpha  and  Beta  Centauri  in 
Centaurus  close  to  the  meridian,  Lupus  due  north  of 
Centaurus,  Ara  and  Triangulum  Australe  in  the  south- 
east and  Crux  in  fine  position  for  observation  just 
west  of  the  meridian.  Carina  of  Argo  Navis  lies 
to  the  southwest  of  Crux.  The  Milky  Way  now 
arches  magnificently  across  the  heavens  from  Carina 
through  Crux,  Centaurus  and  Lupus  and  Ara  to  the 
zodiacal  groups  of  Scorpio  and  Sagittarius  in  the 
northeast. 

In  the  northern  part  of  the  heavens,  as  seen  from 
the  southern  hemisphere,  appear  the  familiar  zodiacal 
constellations  that  we  of  the  northern  hemisphere  find 
south  of  the  zenith,  as  well  as  the  constellations  of 
Orion,  Lepus  and  Canis  Major,  Hydra,  Corvus  and 
Crater,  Ophiuchus  and  Serpens  and  Aquila,  all  finely 
in  view  in  their  appropriate  seasons. 

It  is  only  our  familiar  circumpolar  constellations 
of  the  north — The  Two  Bears,  Draco,  Cassiopeia,  and 
Cepheus,  Andromeda  and  Perseus  and  Auriga  that 
are  invisible  in  mid-latitudes  of  the  southern  hemi- 
sphere just  as  the  constellations  shown  in  the  dia- 
grams, and  a  few  additional  groups  such  as  Pavo, 
Grus,  Phoenix,  Apus,  Mensa  and  Volans  which  we 
have  not  shown,  lie  hidden  from  view  beneath  the 
southern  horizon  in  mid-latitudes  of  the  northern 
hemisphere. 


106      ASTRONOMY  FOR  YOUNG  FOLKS 

The  northern  visitor  to  the  southern  hemisphere 
familiar  with  the  constellations  of  his  own  land  is 
filled  with  a  queer  sensation  of  being  in  topsy-turvydom 
as  he  sees  familiar  Orion  standing  on  his  head  and 
all  of  the  zodiacal  constellations  passing  in  their  daily 
motions  to  the  north  instead  of  to  the  south  of  his 
zenith  while  by  day  the  sun  passes  across  the  northern 
part  of  the  heavens  and  culminates  north  instead  of 
south  of  his  zenith.  He  misses  the  familiar  Dippers 
of  his  own  land  and  searches  in  vain  for  a  pole-star 
in  the  unfamiliar  circumpolar  regions  of  the  south. 


XV 

THE   MILKY   WAY   OR   GALAXY 

"Broad  and   ample   road   whose   dust  is  gold, 
And  pavement  stars,  as  stars  to  thee  appear 
Seen  in  the  galaxy,   that   milky   way 
Which  nightly  as  a   circling  zone  thou  seest 
Powder' d  with  stars." 

— MILTON,  Paradise  Lost. 

ON  clear,  winter  evenings  one  may  see  a  portion 
of  the  zone  of  the  Milky  Way,  which  encircles 
the  heavens,  arching  magnificently  across  the  heavens 
as  it  passes  from  Cassiopeia  and  Cepheus  in  the  north- 
west, through  Perseus  and  Auriga  and  the  eastern 
part  of  Taurus,  across  the  feet  of  Gemini,  between 
Canis  Minor  and  Orion  and  through  the  eastern  part 
of  Canis  Major  to  the  southern  horizon. 

At  this  point  it  passes  beyond  our  range  of  vision 
into  the  star-groups  of  Puppis,  Vela  and  Carina,  sub- 
divisions of  the  huge  southern  constellation  of  Argo 
Navis.  It  reaches  its  greatest  distance  south  of  the 
celestial  equator  and  also  attains  its  greatest  brilliancy 
in  Crux,  the  far-famed  Southern  Cross.  From  here 
it  turns  northward  once  more,  passing  into  Centaurus, 
Musca,  Circinus  Ara  and  Lupus  constellations  of  the 
southern  hemisphere  and  comes  within  our  range  of 
vision  again  in  Sagittarius  and  Scorpio.  Here  the 

107 


108      ASTRONOMY  FOR  YOUNG  FOLKS 

Milky  Way  divides  into  tv;o  branches,  though  some 
astronomers  now  believe  that  this  apparent  division 
into  two  branches  is  due  to  the  presence  of  an 
enormous  cloud  of  non-luminous  matter  lying  along 
the  course  of  the  Milky  Way  at  this  point,  similar 
in  its  nature  to  the  dark  "holes"  and  "caves"  and 
streaks  that  appear  in  all  portions  of  the  Milky  Way 
and  most  noticeably  athwart  its  course  in  Argo  and 
Centaurus. 

One  of  these  branches  of  the  Milky  Way  passes 
from  Sagittarius  through  Aquila  to  Cygnus  and  the 
other  through  Scorpio,  Ophiuchus  and  Serpens  to  Cyg- 
nus, the  two  extending  diagonally  across  the  heavens 
in  the  late  summer  and  early  fall  evenings  from  the 
northeast  to  the  southwest.  From  Cygnus,  the  Milky 
Way  passes  into  Cepheus  and  Cassiopeia  and  thus 
completes  its  circuit  of  the  heavens. 

It  is  not  seen  to  advantage  in  spring  or  early  sum- 
mer evenings  because  it  then  rests  nearly  on  the  hori- 
zon. Its  plane  is  inclined  about  63°  to  the  celestial 
equator  and  its  poles  lie  in  the  constellations  of  Coma 
Berenicis  and  Cetus.  These  are  the  two  points  that 
lie  farthest  from  the  Milky  Way. 

The  Milky  Way  has  been  called  the  groundwork 
of  the  universe.  By  far  the  greater  number  of  all  the 
stars  are  crowded  towards  its  plane  in  the  form  of 
an  enormous  flattened  disk  or  lens. 

Our  solar  system,  it  has  been  estimated,  lies  close 
to  the  plane  of  the  Milky  Way  and  at  a  distance  of 
some  50,000  or  60,000  light-years  from  its  center. 


THE  MILKY  WAY  OR  GALAXY         109 

The  diameter  of  the  Milky  Way  as  deduced  from  Dr. 
Harlow  Shapley's  work  on  globular  star  clusters 
is  about  300,000  light-years  in  extent,  or  ten  times 
greater  than  the  limit  set  some  years  ago. 

The  apparent  crowding  together  of  the  stars  into 
dense  clouds  in  the  Milky  Way  is  partly  an  effect 
due  to  our  position  in  the  Milky  Way.  When  we  look 
at  the  heavens  in  a  direction  at  right  angles  to  this 
plane  we  find  comparatively  few  stars  lying  along 
our  line  of  vision  because  the  stars  are  actually 
fewer  in  number  in  this  direction.  If  we  look  along 
the  plane  of  the  Milky  Way,  however,  we  see  to  a 
greater  distance  through  an  enormous  depth  of  stars. 
Though  individual  stars  may  not  be  much  closer  to- 
gether in  the  Milky  Way  than  they  are  outside  of 
it,  there  are  on  the  whole  more  of  them  and  the  effect 
of  greater  density  is  produced. 

Father  Hagen  of  the  Vatican  Observatory,  who  has 
for  years  made  a  study  of  the  dark  clouds  of  ob- 
scuring matter  and  dark  nebulae  that  abound  in  space, 
has  found  evidence  of  the  existence  of  many  vast  clouds 
of  dark  obscuring  matter  over  the  entire  heavens 
above  and  below  the  plane  of  the  Milky  Way  as  well 
as  surrounding  the  Milky  Way  in  its  own  plane.  The 
existence  of  such  clouds  of  non-luminous  matter  would 
account  partly  for  the  comparative  fewness  of  stars 
in  space  outside  of  the  plane  of  the  Milky  Way  since 
many  stars  would  be  concealed  from  our  eyes  by  these 
obscuring  clouds.  There  is,  however,  in  addition,  an 


110   ASTRONOMY  FOR  YOUNG  FOLKS 

actual   crowding   of   all   the   visible   stars   toward  this 
plane. 

The  peoples  of  all  ages  have  honored  the  Milky 
Way  in  story  and  legend.  It  has  been  universally 
referred  to  as  The  Sky  River  and  The  Pathway  of 
Souls.  To  the  little  Hiawatha,  we  remember,  the 
"wrinkled  old  Nakomis" 

"Showed  the  broad  white  road  in  heaven 
Pathway  of  the  ghosts,   the   shadows, 
Running   straight   across   the   heavens, 
Crowded   with  the  ghosts,   the   shadows. 
To   the    Kingdom    of   Ponemah 
To  the  land  of  the  hereafter." 


In     The    Galaxy,     Longfellow     thus    describes    the 
Milky  Way: 

"Torrent  of  light  and  river  of  the  air 

Along  whose  bed  the  glimmering  stars  are  seen 

Like  gold  and  silver  sands  in  some  ravine 

Where  mountain  streams  have  left  their  channels  bare!" 


In  Sweden,  where  the  Milky  Way  arches  high 
through  the  zenith  in  winter,  it  is  called  the  Winter 
Street,  and  Miss  Edith  Thomas  writes  thus  beautifully 
of  it  in  her  poem  entitled,  "The  Winter  Street"  : 

"Silent  with  star  dust,  yonder  it  lies — 
The  Winter  Street,  so  fair  and  so  white; 

Winding  along  through  the  boundless  skies, 
Down  heavenly  vale,  up  heavenly  height. 


A  DARK  NEBULA  :  THE  DARK  BAY  OR  DARK  HORSE  NEBULA  IN  ORION 
Taken  with  loo-inch  Hooker  Telescope  of  the  Mt.  Wilson  Observatory 


THE  MILKY  WAY  OR  GALAXY         111 

Faintly  it  gleams,  like  a  summer  road 
When  the  light  in  the  west  is  sinking  low, 

Silent  with  star  dust!  By  whose  abode 
Does  the  Winter  Street  in  its  windings  go? 

And  who  are  they,  all  unheard  and  unseen — 

O  who  are  they,  whose  blessed   feet 
Pass  over  that  highway  smooth  and  sheen? 

What   pilgrims   travel   the   Winter   Street? 

Are  they  not  those  whom  here  we  miss 

In   the  ways   and  the  days   that  are  vacant  below? 
As  the  dust  of  that   Street  their  footfalls  kiss 

Does  it  not  brighter  and  brighter  grow?" 


Beautiful  indeed  are  these  poetic  fancies  but  none 
of  them  picture  even  remotely  the  awe-inspiring 
grandeur  of  the  Milky  Way  as  it  actually  exists. 

Millions  upon  millions  of  far  distant  suns  equal  to  or 
surpassing  our  own  sun  in  brilliancy  are  gathered 
within  this  vast  encircling  zone  of  the  heavens,  their 
combined  light  giving  to  the  naked  eye  the  impression 
of  a  milky  band  of  light.  Nine-tenths  of  all  the  stars, 
it  has  been  estimated,  lie  close  to  the  plane  of  the 
Galaxy,  as  well  as  all  the  vast  expanses  of  luminous 
gaseous  nebulae  and  clouds  of  dark  obscuring  matter 
all  seemingly  intermingled  in  chaotic  confusion;  yet 
law  and  order  govern  the  motions  of  all.  Here  also 
are  the  great  moving  star  clusters  such  as  the  Pleiades 
and  the  Hyades  and  all  of  the  brilliant  "Orion"  stars. 

The  structure  of  the  Milky  Way  is  not  clearly  under- 
stood but  many  astronomers  believe  there  is  evidence 


112      ASTRONOMY  FOR  YOUNG  FOLKS 

that  it  takes  the  form  of  a  vast  spiral  nebula  along 
whose  arms  the  stars  pass  to  and  fro. 

Beyond  the  Milky  Way  at  enormous  distances  of 
many  thousands  of  light-years,  but  apparently  in- 
fluenced by  it,  lie  the  globular  star-clusters  and  the 
spiral  nebulae.  The  spirals  appear  to  avoid  the  plane 
of  the  Milky  Way  for  they  are  receding  in  the  di- 
rection of  its  poles  at  high  velocities;  the  globular  clus- 
ters on  the  other  hand  are  drawing  in  toward  the 
Milky  Way  on  either  side,  and  in  time  will  cross  it. 

Whether  these  objects  external  to  the  Milky  Way 
form  with  it  one  enormous  universe  or  whether  the 
spiral  nebulae  are  in  turn  galaxies  or  "island  universes," 
as  the  astronomer  calls  them,  similar  in  form  and 
structure  to  our  own  galaxy  and  at  inconceivably  great 
distances  of  millions  of  light-years  from  it,  is  still 
one  of  the  riddles  of  the  universe  which  the  astrono- 
mers are  attempting  to  solve. 


XVI 

THE  SURFACE  OF  THE  SUtf 

THE  visible  surface  of  the  sun  is  called  the  photo- 
sphere.  Even  the  smallest  telescopes  will  show  its 
peculiar  "rice-grain"  structure,  consisting  of  intensely 
brilliant  flecks  or  nodules  about  500  miles  in  diameter, 
which  can  be  resolved  by  the  more  powerful  telescopes 
into  smaller  particles  about  100  miles  in  diameter, 
against  a  darker  background.  It  has  been  estimated 
that  these  bright  nodules  or  rice-grains  occupy  only 
one-fifth  of  the  total  surface  of  the  sun,  yet  radiate 
three- fourths  of  the  total  light. 

It  is  generally  believed  that  the  "rice  grains"  are 
the  summits  of  highly  heated  columns  of  gas,  arising 
from  the  sun's  interior,  and  that  the  darker  portions 
between  are  cooler  descending  currents. 

It  is  well  known  that  the  photosphere  or  visible 
surface  of  the  sun  appears  to  be  much  brighter  in  the 
center  of  the  disk  than  near  its  circumference.  This  is 
due  entirely  to  its  gaseous  nature  and  to  the  fact  that 
it  is  surrounded  by  an  atmosphere  of  dense  enveloping 
cooler  gases.  Rays  from  the  center  of  the  disk  travel 
a  shorter  distance  through  this  atmosphere  than  the  rays 
from  the  rim  and  therefore  are  absorbed  less  by  sur- 
rounding gases.  We  look  further  down  into  the  sun's 

113 


114      ASTRONOMY  FOR  YOUNG  FOLKS 

interior  near  the  center  of  the  disk  than  in  the  direc- 
tion of  its  circumference  and  so  the  light  appears 
more  intense  there. 

The  photosphere  is  the  region  where  sun-spots  appear 
and  they  are  found  in  zones  extending  from  8°  to  35° 
on  either  side  of  the  solar  equator,  never  appearing  ex- 
actly at  the  equator  or  near  the  poles. 

The  disturbances  that  produce  sun-spots  and  many 
allied  phenomena  occur  cyclically  in  periods  of  eleven 
years  on  the  average.  The  first  outburst  of  the  dis- 
turbance is  manifested  by  the  appearance  of  sun-spots 
in  high  solar  latitudes.  These  break  out  and  disap- 
pear and  break  out  again  with  increased  vigor,  working 
gradually  downward  toward  the  solar  equator,  the 
maximum  spottedness  for  a  given  period  occurring  in 
solar  latitude  about  16°.  The  disturbance  finally  dies 
out  within  8°  or  10°  of  the  equator,  but  even  before 
one  cycle  of  disturbance  has  entirely  passed  away  a 
new  cycle  has  broken  forth  in  high  latitudes.  So  dur- 
ing the  period  of  minimum  spottedness  there  are  four 
distinct  belts,  two  in  low  latitudes,  due  to  the  dying  dis- 
turbance, and  two  in  high  latitudes,  due  to  the  new  dis- 
turbance. At  sun-spot  maximums  there  are  two  well- 
marked  zones  of  great  intensity,  approximately  16° 
north  and  south  of  the  sun's  equator. 

Sun-spots  are  solar  cyclones,  occurring  usually  in 
groups,  though  large  single  spots  appear  less  frequently. 
Each  spot  is  quite  sharply  divided  into  an  umbra  and  a 
penumbra.  The  umbra  is  the  darker  central  portion, 
the  funnel  of  the  whirling  cyclone,  and  the  penumbra  is 


THE  SURFACE  OF  THE  SUN  115 

composed  of  the  outspreading  gases,  and  is  less  dark 
than  the  umbra.  The  peculiar  "thatch-straw"  structure 
of  the  penumbra  is  due,  it  is  believed,  to  the  fact  that 
the  columns  of  gases  that  usually  rise  vertically  from 
the  sun's  interior  and  from  the  "rice  grains"  of  the 
photosphere  are  drawn  into  a  horizontal  position  by 
the  whirling  motion  that  exists  in  the  penumbra  regions 
of  a  sun-spot  and  therefore  we  get  a  longitudinal  rather 
than  a  cross  sectional  view  of  them. 

The  umbra  of  a  sun-spot  is  anywhere  from  a  few 
hundred  miles  to  fifty  thousand  miles  in  diameter,  fre- 
quently exceeding  the  earth  in  size,  while  the  penumbra 
occasionally  reaches  a  diameter  of  two  hundred  thou- 
sand miles.  Sun-spots  of  exceptional  size  can  be  seen 
even  without  the  aid  of  a  telescope. 

The  darkness  of  sun-spots  is  only  by  comparison  with 
their  more  brilliant  background.  Owing  to  the  rapid 
expansion  and  cooling  of  gases  the  temperature  in  sun- 
spot  regions  is  far  below  the  normal  solar  temperature 
of  6,000°  Centigrade,  lying  between  3,000°  and  4,000° 
Centigrade.  At  this  temperature  it  is  possible  for  the 
more  refractory  chemical  compounds  to  form,  the  ox- 
ides and  the  hydrides,  and  the  spectra  of  sun-spots  re- 
veal the  presence  of  titanium  oxide  and  magnesium 
and  calcium  hydride.  At  the  higher  solar  temperatures 
that  exist  elsewhere  in  the  photosphere  and  in  its  over- 
lying gaseous  envelopes  all  chemical  elements  occur  in  a 
free  state,  intermingling  as  incandescent  vapors  without 
the  formation  of  any  chemical  compounds. 

Strong  magnetic  fields  exist  in  sun-spot  regions  and 


116      ASTRONOMY  FOR  YOUNG  FOLKS 

magnetic  storms  in  our  own  atmosphere  frequently  ac- 
company the  appearance  of  exceptionally  large  sun- 
spots. 

Directly  above  the  photosphere  of  the  sun  lies  the 
"reversing  layer,"  which  is  about  five  hundred  miles 
in  depth  and  is  composed  of  the  incandescent  vapors 
of  all  the  chemical  elements  that  exist  on  the  sun,  which 
are  also  the  same  familiar  elements  that  exist  on  the 
earth,  with  the  exception  of  coronium,  the  unknown 
element  in  the  solar  corona,  there  is  no  element  in  the 
sun  that  has  not  been  found  on  our  own  planet. 

The  "reversing  layer"  receives  its  name  from  the 
fact  that  it  reverses  the  solar  spectrum.  It  produces 
by  its  absorption  of  the  rays  of  light  from  gases  below 
the  dark  absorption  lines  found  in  the  spectrum  that 
serve  to  identify  all  the  elements  existing  in  the  sun. 
During  the  time  immediately  preceding  and  following 
a  total  eclipse  of  the  sun  this  reversing  layer  produces 
what  is  known  as  the  flash  spectrum.  When  the  photo- 
sphere, which  gives  the  bright  continuous  background 
of  the  solar  spectrum,  is  concealed  by  the  moon,  the 
normally  dark  lines  of  the  reversing  layer — dark  only 
by  contrast  with  the  bright  background — become  mo- 
mentarily intensely  bright  lines  against  a  dark  back- 
ground. The  flash  spectrum  only  lasts  a  second  or 
so,  as  the  reversing  layer  itself  is  soon  covered  by  the 
moon. 

Just  above  the  reversing  layer  lies  the  chromosphere, 
which  is  between  five  thousand  and  ten  thousand  miles 
in  depth.  Many  of  the  gaseous  vapors  of  the  reversing 


THE  SURFACE  OF  THE  SUN  117 

layer  are  found  in  the  chromosphere,  thrown  there  con- 
tinually by  the  vast  upheavals  of  gases  that  are  con- 
stantly disturbing  the  surface  of  the  sun.  The  greater 
the  solar  activity  the  more  is  the  chromosphere  charged 
with  the  vapors  of  the  lower  strata  of  the  sun's  atmo- 
sphere. The  gases  that  are  most  characteristic  of  the 
chromosphere,  however,  are  the  incandescent  gases  of 
hydrogen  and  calcium,  which  give  it  the  pink  or  reddish 
tinge  so  noticeable  during  total  solar  eclipse.  Helium  is 
also  found  in  great  abundance  in  the  solar  chromo- 
sphere. 

Shooting  upward  from  the  photosphere  with  the 
tremendous  velocity  of  one  hundred  or  more  miles  per 
second,  can  be  seen  at  all  times,  by  properly  screening 
off  the  light  from  the  photosphere,  the  vast  solar  erup- 
tions known  as  the  prominences.  These  are  composed 
chiefly  of  hydrogen  and  calcium  gas,  though  other  ele- 
ments also  appear,  especially  near  the  bases  of  the  prom- 
inences. Prominences  are  of  two  varieties,  the  quiescent, 
or  cloud-like  prominences,  that  float  high  above  the  solar 
surface  for  days  at  a  time  in  some  instances  and  resem- 
ble terrestrial  clouds  in  form,  and  the  eruptive,  or  metal- 
lic prominences,  that  dart  up  from  the  surface  of  the 
sun  in  an  infinite  variety  of  forms  that  may  be  entirely 
changed  in  the  short  interval  of  fifteen  or  twenty 
minutes. 

These  eruptive  prominences  usually  attain  heights  of 
thirty  or  forty  thousand  miles  on  the  average,  but  ex- 
ceptional prominences  reach  heights  of  more  than  one 
hundred  thousand  miles  and  in  a  few  rare  cases  have 


118      ASTRONOMY  FOR  YOUNG  FOLKS 

reached  elevations  of  over  five  hundred  thousand  miles, 
or  more  than  one-half  of  the  solar  diameter. 

Prominences  are  the  most  spectacular  and  beautiful 
of  all  solar  phenomena,  with  the  possible  exception  of 
the  solar  corona,  which  is  the  outermost  of  all  the  solar 
envelopes  and  also  the  most  tenuous.  The  extent  of 
the  corona  is  enormous.  Its  outer  streamers  extend 
usually  to  distances  of  several  million  miles  from  the 
center  of  the  sun.  Measurements  of  the  coronal  light 
during  total  eclipses  of  the  sun  have  shown  that  its 
intensity  is  only  about  one-half  that  of  full  moonlight, 
and  it  seems  almost  impossible  to  devise  methods  for 
detecting  it,  except  during  total  eclipses,  on  account  of 
the  extreme  faintness  of  its  light. 

The  sun,  it  is  now  known,  is  surrounded  by  a  strong 
magnetic  field  in  addition  to  the  magnetic  fields  that  exist 
in  sun-spots.  The  cycle  of  sun-spot  change  is  at- 
tended by  marked  changes  in  many  forms  of  solar 
activity.  The  frequency  of  outbursts  of  eruptive  promi- 
nences, the  brightness  and  form  of  the  corona,  mag- 
netic storms  and  weather  changes  on  the  earth  are  all 
closely  associated  with  the  sun-spot  cycle. 

The  cause  of  this  sun-spot  cycle,  with  all  the  at- 
tendant changes  in  the  general  solar  activity,  and  the 
source  of  the  apparently  limitless  supply  of  solar  energy 
still  remain  the  two  chief  unsolved  secrets  of  the  sun. 


XVII 
THE  SOLAR  SYSTEM 

OUR  sun  is  but  a  star  traveling  through  the  universe. 
'It  is  accompanied  in  its  journey  to  unknown  parts 
of  space,  that  lie  in  the  general  direction  of  the  constel- 
lation Hercules,  by  an  extensive  family  of  minor  bodies 
consisting  of  the  eight  planets  and  their  encircling 
moons,  one  thousand  or  more  asteroids,  numerous  com- 
ets, and  meteors  without  number,  all  moving  in  pre- 
scribed paths  around  their  ruler. 

The  most  important  members  of  the  sun's  family  are 
the  planets,  Mercury,  Venus,  Earth,  Mars,  Jupiter, 
Saturn,  Uranus  and  Neptune,  named  in  the  order  of 
their  position  outward  from  the  sun.  We  hear  occasion- 
ally of  the  possibility  of  the  existence  of  intra-Mercurial 
and  trans-Neptunian  planets  and  it  is  possible  that  some 
day  an  additional  planet  may  be  discovered  within  the 
orbit  of  Mercury  or  beyond  the  orbit  of  Neptune.  The 
gravitational  control  of  the  sun  extends  far  beyond  the 
orbit  of  Neptune  and  there  are  reasons  for  believing  in 
the  existence  of  at  least  one  or  two  additional  planets 
on  the  outskirts  of  the  solar  system.  The  existence  of 
a  planet  within  the  orbit  of  Mercury  is  now,  after  long 
continued  and  diligent  search,  believed  to  be  very 
doubtful. 

119 


120   ASTRONOMY  FOR  YOUNG  FOLKS 

Were  it  possible  to  view  the  sun  from  the  distance 
of  the  nearest  star  with  the  aid  of  the  greatest  telescope 
on  earth  all  the  members  of  his  family  would  be  hope- 
lessly invisible.  So,  also,  we  cannot  tell  as  we  point  our 
powerful  telescopes  at  the  stars  whether  these  other  suns 
are  attended  by  planet  families.  We  may  only  argue 
that  it  is  very  unlikely  that  there  should  be  only  one 
star  among  hundreds  of  millions  that  is  attended  by 
a  group  of  comparatively  small  dark  bodies  that  shine 
by  the  reflected  light  from  the  star  they  encircle. 

With  the  exception  of  the  two  planets,  Mercury  and 
Venus,  which  are  known  as  the  inferior  planets,  since 
their  paths  lie  between  the  earth  and  the  sun,  all  the 
planets  have  moons  or  satellites  of  their  own  that  encircle 
the  planet  just  as  the  planet  encircles  the  sun. 

Our  planet  earth  has  one  satellite,  the  moon,  that 
has  the  distinction  of  being  the  largest  of  all  the  moons 
in  proportion  to  the  size  of  the  planet  it  encircles. 
Jupiter  and  Saturn  have  moons  that  surpass  our  moon  in 
actual  size;  in  fact,  two  of  the  moons  of  the  outer 
planets  are  actually  larger  than  the  smallest  planet  Mer- 
cury, but  they  are  very  small  in  proportion  to  the  size  of 
the  planets  around  which  they  revolve.  Mars,  the  next 
planet  beyond  the  earth,  the  nearest  of  the  superior  or 
outer  planets,  has  two  tiny  mdons  that  bear  the  names 
of  Deimos  and  Phobos,  respectively.  They  are  both 
less  than  twenty  miles  iru  diameter  and  revolve  very 
near  to  the  surface  of  Mars.  They  can  only  be  seen 
with  the  aid  of  very  powerful  telescopes.  The  inner 
moon,  Phobos,  is  unique  in  the  solar  system  for  it  makes 


THE  SOLAR  SYSTEM  121 

three  trips  around  Mars  while  the  planet  is  turning  once 
on  its  axis. 

Jupiter,  the  next  planet  outward  from  the  sun,  is  al- 
most a  sun  itself  to  its  extensive  family  of  nine  moons. 
Four  of  these  moons  were  first  seen  about  three  hundred 
years  ago  when  Galileo  pointed  his  first  crude  telescope 
at  the  heavens  and  any  one  can  now  see  them  with  the 
aid  of  an  opera  glass.  One  of  the  four  is  equal  in  size 
to  our  own  moon;  the  others  surpass  it  in  size.  These 
moons  are  most  interesting  little  bodies  to  observe.  Their 
eclipses  in  the  shadow  of  Jupiter,  occupations  or  disap- 
pearances behind  his  disk,  and  the  transits  of  the  shad- 
ows as  well  as  the  moons  themselves  across  the  face  of 
the  planet  can  be  easily  seen  even  with  the  smallest  tele- 
scope. The  five  remaining  moons  have  all  been  dis- 
covered in  modern  times.  They  are  extremely  small 
bodies  visible  only  in  large  telescopes.  Satellite  V  is  the 
nearest  of  all  the  moons  to  Jupiter.  The  other  four 
are  at  great  distances  from  the  planet. 

The  planet  Saturn  has  nine  moons.  Titan,  the  largest, 
is  nearly  equal  in  size  to  Jupiter's  largest  moon,  and  is 
larger  than  Mercury;  four  of  the  other  moons  have 
diameters  between  one  thousand  and  two  thousand 
miles  in  length.  Since  Saturn  is  nearly  twice  as  far 
from  the  sun  as  Jupiter  his  moons  are  more  difficult  to 
observe,  though  the  two  largest  are  visible  in  small 
telescopes. 

Saturn  is  unique  in  the  solar  system  in  possessing  in 
addition  to  his  nine  satellites  a  most  wonderful  ring 
system,  composed  of  swarms  of  minute  moonlets,  each 


122      ASTRONOMY  FOR  YOUNG  FOLKS 

pursuing  its  individual  path  around  the  planet.  It  is 
this  unusual  ring  system  that  makes  Saturn  the  most  in- 
teresting to  observe  telescopically  of  all  the  planets. 

The  planet  Uranus  has  four  satellites  and  Neptune 
one.  These  planets  and  their  satellites  cannot  be 
well  observed  on  account  of  their  great  distances  from 
the  earth.  The  indistinctness  of  surface  markings 
makes  it  impossible  to  determine  the  period  of  rotation 
of  these  two  outer  planets  on  their  axes.  It  is  believed 
that  their  rotation  is  very  rapid,  however,  as  is  the 
case  with  the  other  planets  Jupiter  and  Saturn. 

All  the  planets  in  the  solar  system  fall  naturally  into 
two  groups.  Jupiter,  Saturn,  Uranus,  and  Neptune,  the 
members  of  the  outer  group,  have  on  the  average,  diam- 
eters ten  times  as  great  and,  therefore,  volumes  one 
thousand  times  as  great  as  Mercury,  Venus,  Earth  and 
Mars,  the  members  of  the  inner  or  terrestrial  group. 

The  terrestrial  planets  are  the  pigmies  of  the  solar 
system,  the  outer  planets  are  the  giants.  The  densities 
of  the  planets  Mercury,  Venus,  Earth  and  Mars  are 
several  times  greater  than  the  density  of  water.  They 
are  all  extremely  heavy  bodies  for  their  size,  and  prob- 
ably have  rigid  interiors  with  surface  crusts. 

The  existence  of  life  on  Mercury  is  made  impossible 
by  the  absence  of  an  atmosphere.  Venus  and  Mars 
both  have  atmospheres  and  it  is  possible  that  both  of 
these  planets  may  support  life.  Mars  has  probably  been 
the  most  discussed  of  all  the  planets,  though  Venus  is 
the  Earth's  twin  planet  in  size,  mass,  density  and  sur- 
face gravity,  just  as  Uranus  and  Neptune  are  the  twins 


A.  VENUS.     B.  MARS.     C.  JUPITER.     D.  SATURN. 

Taken  by  Prof.  E.  E.  Barnard  with  the  4O-inch  telescope  of  the  Yerkes 

Observatory,  with  exception  of   Saturn,  which  was  taken  by   Prof. 

Barnard  on  Mt.  Wilson 

(See  note  page  126) 


THE  SOLAR  SYSTEM  123 

of  the  outer  group.  It  is  now  believed  that  water  and 
vegetation  exist  on  Mars.  The  reddish  color  of  this 
planet  is  supposed  to  be  due  to  its  extensive  desert 
tracts.  The  nature  of  certain  peculiar  markings  on  this 
planet,  known  as  canals,  still  continues  to  be  a  matter  of 
dispute.  It  is  generally  believed  since  air,  water  and 
vegetation  exist  on  Mars,  that  some  form  of  animal  life 
also  exists  there. 

The  length  of  the  day  on  Mars  is  known  very  accu- 
rately, for  the  rareness  of  its  atmosphere  permits  us 
to  see  readily  many  of  its  surface  markings.  The  length 
of  the  day  is  about  twenty- four  and  one-half  hours,  and 
the  seasonal  changes  on  Mars  strongly  resemble  our 
own,  though  the  seasons  on  Mars  are  twice  as  long 
as  they  are  on  our  own  planet  since  the  Martian  year  is 
twice  as  long  as  the  terrestrial  year. 

The  question  of  life  on  Venus  depends  largely  upon 
the  length  of  the  planet's  rotation  period.  This  is  still 
uncertain  since  no  definite  surface  markings  can  be 
found  on  the  planet  by  which  the  period  of  its  rotation 
can  be  determined.  So  dense  is  the  atmosphere  of 
Venus  that  its  surface  is,  apparently,  always  hidden 
from  view  beneath  a  canopy  of  clouds.  It  is  the  more 
general  belief  that  Venus,  as  well  as  Mercury,  rotates 
on  its  axis  in  the  same  time  that  it  takes  to  make  a 
revolution  around  the  sun.  In  this  case  the  same  side 
of  the  planet  is  always  turned  toward  the  sun  and,  as 
a  result,  the  surface  is  divided  into  two  hemispheres — 
one  of  perpetual  day,  the  other  of  perpetual  night. 

This  peculiar  form  of  rotation  in  which  the  period 


124      ASTRONOMY  FOR  YOUNG  FOLKS 

of  rotation  and  revolution  are  equal  is  by  no  means  un- 
known in  the  solar  system.  Our  own  moon  always 
keeps  the  same  face  turned  toward  the  earth  and  there 
are  reasons  for  believing  that  some  of  the  satellites 
of  Jupiter  and  Saturn  rotate  in  the  same  manner. 

Life  on  any  one  of  the  outer  planets  is  impossible. 
The  density  of  these  planets  averages  about  the  same 
as  the  density  of  the  sun,  which  is  a  little  higher  than 
the  density  of  water.  The  density  of  Saturn  is  even 
less  than  water.  In  other  words,  Saturn  would  float 
in  water  and  it  is  the  lightest  of  all  the  planets.  It 
is  assumed  from  these  facts  that  the  four  outer  planets 
are  largely  in  a  gaseous  condition.  They  all  possess 
dense  atmospheres  and,  in  spite  of  their  huge  size, 
rotate  on  their  axis  with  great  rapidity.  The  two 
whose  rotation  periods  are  known,  Jupiter  and  Saturn, 
turn  on  their  axis  in  about  ten  hours.  On  account  of 
this  rapid  rotation  and  their  gaseous  condition  both 
Jupiter  and  Saturn  are  noticeably  flattened  at  the  poles. 

The  terrestrial  planets  are  separated  from  the  outer 
group  by  a  wide  gap.  Within  this  space  are  to  be 
found  the  asteroid  or  planetoid  group.  There  are 
known  to  be  over  nine  hundred  and  fifty  of  these  minor 
bodies  whose  diameters  range  from  five  hundred  miles 
for  the  largest  to  three  or  four  miles  for  the  smallest. 
There  are  only  four  asteroids  whose  diameters  exceed 
one  hundred  miles  and  the  majority  have  diameters  of 
less  than  twenty  miles.  The  total  mass  of  the  asteroids 
is  much  less  than  that  of  the  smallest  of  the  planets.  It 
was  believed  at  one  time  that  these  small  bodies  were 


THE  SOLAR  SYSTEM  125 

fragments  of  a  shattered  planet,  but  this  view  is  no 
longer  held.  The  asteroids  as  well  as  the  comets  and 
meteors  probably  represent  the  material  of  the  primi- 
tive solar  nebula  that  was  not  swept  up  when  the 
larger  planets  were  formed. 

With  few  exceptions  the  asteroids  are  only  to  be 
seen  in  large  telescopes  and  then  only  as  star-like  points 
of  light.  Most  of  them  are  simply  huge  rocks  and  all 
are  necessarily  devoid  of  life  since  such  small  bodies 
have  not  sufficient  gravitational  force  to  hold  an 
atmosphere. 

The  revolution  of  the  planets  around  the  sun  and  of 
the  satellites  of  the  planets  around  the  primary  planets 
are  performed  according  to  known  laws  of  motion  that 
make  it  possible  to  foretell  the  positions  of  these  bodies 
years  in  advance.  Asteroids  and  comets  also  obey  these 
same  laws,  and  after  three  observations  of  the  positions 
of  one  of  these  bodies  have  been  obtained  its  future  move- 
ments can  be  predicted.  All  the  planets  and  their  satel- 
lites are  nearly  perfect  spheres.  They  all,  with  few  ex- 
ceptions, rotate  on  their  axes  and  revolve  around  the  sun, 
or,  in  the  case  of  moons,  around  their  primaries,  in  the 
same  direction,  from  west  to  east.  Only  the  two  outer- 
most satellites  of  Jupiter,  the  outermost  satellites  of  Sa- 
turn and  the  satellites  of  Uranus  and  Neptune  retro- 
grade or  travel  in  their  orbits  from  east  to  west,  which  is 
opposite  to  the  direction  of  motion  of  all  the  other  plan- 
ets and  satellites. 

The  paths  of  all  the  planets  around  the  sun  are 
ellipses  that  are  nearly  circular,  and  they  all  lie  nearly 


126      ASTRONOMY  FOR  YOUNG  FOLKS 

in  the  same  plane.  The  asteroids  have  orbits  that  are 
more  flattened  or  elliptical  and  these  orbits  are  in  some 
instances  highly  inclined  to  the  planetary  orbits.  The 
comets  have  orbits  that  are  usually  very  elongated  el- 
lipses or  parabolas.  Some  of  the  comets  may  be  only 
chance  visitors  to  our  solar  system,  though  astronomers 
generally  believe  that  they  are  all  permanent  members. 
Paths  of  comets  pass  around  the  sun  at  all  angles  and 
some  comets  move  in  their  orbits  from  west  to  east, 
while  others  move  in  the  opposite  direction  or  retrograde. 
The  behavior  of  the  asteroids  and  comets  is  not  at  all  in 
accord  with  the  theory  that  was,  until  recently,  uni- 
versally advanced  to  explain  the  origin  of  the  solar 
system. 

Some  astronomers  have  made  attempts  to  modify  the 
nebular  hypothesis  that  has  held  sway  for  so  many 
years,  in  order  to  make  it  fit  in  with  more  recent  dis- 
coveries, but  others  feel  that  a  new  theory  is  now  re- 
quired to  explain  the  origin  of  the  solar  system.  Sev- 
eral theories  have  been  advanced  but  no  new  theory  has 
yet  definitely  replaced  the  famous  nebular  hypothesis 
of  the  noted  French  astronomer  La  Place. 

NOTE:  The  reader  must  bear  in  mind  that  the  telescopic  views 
of  the  four  planets  have  not  been  reduced  to  the  same  scale  and 
so  are  not  to  be  compared  in  size. 


XVIII 
THE  ORIGIN  OF  THE  EARTH 

IT  is  not  possible  to  consider  the  question  of  the 
origin  of  the  earth  apart  from  the  question  of  the  or- 
igin of  the  solar  system.  That  all  the  planets,  as  well 
as  the  asteroids,  originated  from  a  common  parent- 
mass  has  never  been  seriously  questioned.  All  of  these 
bodies  revolve  about  the  sun,  and  rotate  upon  their 
axes  in  the  same  direction — from  west  to  east.  More- 
over, all  of  the  planetary  orbits  lie  very  nearly  in  the 
same  plane  and  are  nearly  circular  in  form. 

The  orbits  of  the  asteroids  are  more  elliptical  and  more 
highly  inclined  to  one  another  than  are  the  orbits 
of  the  planets,  but  on  the  average  they  are  neither  very 
elliptical  nor  very  highly  inclined  to  the  planetary  orbits. 

The  sun  rotates  upon  its  axis  in  the  same  direction 
in  which  the  planets  rotate  and  perform  their  revolu- 
tions, and  the  orbits  of  the  planets  are  inclined  at  small 
angles  to  the  plane  of  the  sun's  equator. 

These  facts  are  all  significant  and  cannot  be  over- 
looked in  formulating  a  theory  to  explain  the  origin 
of  the  planetary  system  in  general  and  of  the  earth 
in  particular.  Presumably  the  planets  and  asteroids 
formed  at  one  time  a  part  of  a  central  body  which  ro- 
tated on  its  axis  in  the  direction  in  which  they  now 
revolve  about  the  sun. 

127 


128       ASTRONOMY  FOR  YOUNG  FOLKS 

When  and  by  the  operation  of  what  force,  external 
or  internal,  they  were  separated  from  this  central  body 
is  the  question. 

In  1796  La  Place  advanced  his  celebrated  nebular 
hypothesis  to  explain  the  origin  of  the  solar  system.  It 
was  received  with  favor  both  by  scientists  and  laymen, 
and  in  a  short  time  was  almost  universally  accepted  as 
closely  approximating  to  the  truth. 

According  to  the  nebular  hypothesis  the  solar  nebula 
from  which  the  planetary  system  was  formed,  originally 
extended  at  least  as  far  as  the  orbit  of  Neptune  and 
rotated  slowly  in  the  direction  in  which  the  planets  now 
revolve.  As  it  lost  heat  by  radiation  and  contracted  un- 
der the  gravitation  of  its  parts  its  rate  of  rotation  in- 
creased. When  the  centrifugal  (center-fleeing)  force 
at  the  equator  equalled  the  gravitational  force  directed 
toward  the  center,  a  ring  would  be  left  behind  by  the 
contracting  nebula.  Such  a  ring  would  not  be  absolutely 
uniform  and  would  break  at  some  point  and  gather  into 
a  planetary  mass  under  the  gravitation  of  its  parts.  This 
planetary  mass  would  abandon  rings  in  turn  and  these 
would  break  up  to  form  satellites.  Successive  rings 
were  supposed  to  have  been  abandoned  at  intervals  by 
the  solar  nebula  at  the  present  distances  of  the  planets 
from  the  sun  in  the  manner  described  above  until  the 
original  solar  nebula  had  contracted  to  its  present  size. 

The  rings  of  Saturn  were  supposed  to  be  the  single 
example  remaining  of  this  process  of  forming  planets 
and  satellites  from  a  contracting  nebulous  mass. 

The  La  Placian  hypothesis  attempted  to  explain  why 


THE  ORIGIN  OF  THE  EARTH  129 

all  the  planets  and  their  satellites  revolve  in  the  same 
direction  in  which  the  sun  turns  on  its  axis,  in  nearly 
circular  orbits  and  nearly  in  the  same  plane.  At  the 
time  it  was  advanced  it  appeared  to  be  in  accord  with 
all  the  facts  then  known  regarding  the  solar  system. 

The  planetoids  with  their  interlacing  and  in  some  in- 
stances highly  inclined  and  elliptical  orbits  were  then 
undiscovered.  It  would  have  been  impossible  for  them 
to  have  been  formed  by  the  abandonment  of  successive 
rings  from  a  central,  rotating  mass. 

The  constitution  of  Saturn's  rings  was  unknown  at 
this  time;  also  the  fact  that  the  moonlets  of  the  inner 
ring  revolve  about  Saturn  in  half  the  time  required  for 
the  planet  to  turn  on  its  axis — another  impossibility 
under  the  nebular  hypothesis,  for,  according  to  the 
assumptions  of  the  nebular  hypothesis  it  would  be  im- 
possible for  a  satellite  to  revolve  about  a  central  body 
in  a  shorter  time  than  that  body  turns  on  its  axis. 

The  satellites  of  Mars  were  not  discovered  until  many 
years  later,  as  well  as  the  retrograding  satellites  of 
Jupiter  and  Saturn,  all  presenting  difficulties  in  the  way 
of  accepting  the  nebular  hypothesis  without  radical 
changes.  Attempts,  mostly  unsuccessful,  have  been 
made  from  time  to  time  to  make  these  exceptional  cases 
fit  in  with  the  requirements  of  the  nebular  hypothesis. 

The  theory  that  the  sun's  heat  was  maintained  by 
the  contraction  of  the  original  solar  nebula,  which 
would  cause  its  temperature  to  rise,  appeared  to  give 
considerable  support  to  the  theory  of  La  Place,  but  the 
mathematicians  got  to  work  and  showed  that  the  amount 


130      ASTRONOMY  FOR  YOUNG  FOLKS 

of  heat  that  would  be  furnished  by  the  contraction  of 
the  sun  from  beyond  the  orbit  of  Neptune  to  its  present 
dimensions  would  be  sufficient  to  supply  heat  to  the 
earth  at  the  present  rate  for  only  twenty-five  million 
years,  a  period  far  too  brief,  the  geologists  and  biolo- 
gists said,  to  cover  all  the  vast  cyclical  changes  that 
are  known  to  have  taken  place  upon  the  surface  of  this 
planet  since  its  surface  crust  was  formed.  Evidently 
gravitational  contraction  is  by  no  means  the  only  or 
even  the  chief  source  of  the  sun's  heat. 

It  was  also  shown  indisputably,  that  it  would  have 
been  impossible  for  successive  rings  to  have  been  aban- 
doned at  certain  definite  intervals  by  a  contracting 
nebula,  and  granted  a  ring  could  have  been  formed  it 
would  have  been  impossible  for  it  to  condense  into  a 
planet,  since  forces  residing  in  the  sun  would  offset 
the  gravitation  of  its  parts. 

When  La  Place  advanced  his  famous  theory  it  was, 
to  use  his  own  words,  "with  that  distrust  which  every- 
thing ought  to  inspire  that  is  not  a  result  of  observa- 
tion or  of  calculation." 

Were  La  Place  living  today  he  would  be,  we  believe, 
the  first  to  abandon  a  theory  that  is  now  known  to 
be  in  accord  neither  with  observation  nor  calculation. 

Deprived  of  a  theory  that  has  served  to  explain  the 
outstanding  features  of  the  solar  system  more  or  less 
adequately  for  one  hundred  and  twenty-five  years,  as- 
tronomers are  seeking  in  the  light  of  recent  observations 
and  discoveries  to  formulate  a  satisfactory  theory  of 
the  origin  of  the  solar  system. 


THE  ORIGIN  OF  THE  EARTH  131 

In  the  planetesimal  theory  of  Chamberlin  and  Moul- 
ton  and  the  recently  suggested  theory  of  the  well-known 
English  mathematician,  Jeans,  a  second  sun  passing 
close  to  our  own  sun  is  assumed  to  have  been  the  cause 
of  the  origin  of  the  planetary  system. 

The  effect  of  the  close  approach  of  such  a  sun  would 
be  the  ejection  of  a  stream  of  matter  from  our  sun,  as 
we  term  it,  in  the  direction  of  the  passing  body  and  also 
in  a  diametically  opposite  direction.  This  ejection 
would  be  continuous  as  long  as  the  stars  remained  near 
one  another,  the  height  attained  by  the  ejected  stream 
decreasing  as  the  passing  star  receded.  The  result  would 
be  the  formation  of  a  spiral  nebula  in  which  the  motion 
of  the  ejected  particles — planetesimals — would  be  across 
the  spiral  arms,  toward  and  away  from  the  passing 
star.  After  the  sun  had  receded  so  far  as  to  have  no 
further  effect  upon  these  ejected  particles  they  would  re- 
volve about  the  sun  in  more  or  less  elliptical  orbits 
which  would  gradually  be  reduced  to  nearly  circular 
forms  by  repeated  collisions  between  planetesimals. 
Larger  nuclei  would  be  formed  and  these  would  gradu- 
ally sweep  up  smaller  fragments  and  become  the  planets 
of  the  present  system.  Smaller  nuclei  in  the  vicinity  of 
larger  ones  would  become  their  satellites  and  in  the 
course  of  many  millions  of  years  all  of  the  larger  frag- 
ments would  be  swept  up  by  the  planetary  nuclei  and 
their  satellites — leaving  only  the  asteroids,  comets  and 
meteors  as  survivors  of  the  original  spiral  system. 

It  must  be  borne  in  mind  that  a  spiral  nebula  formed 
by  the  close  approach  of  two  suns  would  resemble  in 


132      ASTRONOMY  FOR  YOUNG  FOLKS 

form  only  the  great  spiral  nebulae  that  are  known  to 
exist  by  hundreds  of  thousands  in  the  heavens.  These 
are  far  too  extensive  to  form  anything  so  small  as  a 
single  solar  system,  but  might  condense  into  sys- 
tems composed  of  many  suns — either  galaxies  or  star 
clusters. 

Jean's  suggested  theory  of  the  origin  of  the  planetary 
system  differs  in  its  details  from  the  above,  though  a 
passing  sun  is  assumed  to  be  the  disturbing  force  that 
causes  the  ejection  of  a  stream  of  matter  which  con- 
denses to  form  the  planets  and  their  satellites.  The 
origin  of  the  inner  planets  is  left  greatly  in  doubt  by 
this  theory,  however,  and  the  system  which  interests 
us  chiefly — the  earth-moon  system — is  the  one  about 
which  it  is  most  difficult  to  arrive  at  any  definite  con- 
clusion. Our  own  sun,  it  is  assumed,  was  dark  and 
cold,  of  low  density  and  with  a  diameter  about  equal 
to  that  of  Neptune's  orbit  at  the  time  of  the  catastrophe 
which  is  placed  at  some  300,000,000  years  ago.  In 
Jean's  words,  "...  The  time  for  arriving  at  con- 
clusions in  cosmogony  has  not  yet  come — and  it  must 
be  left  to  future  investigators  armed  with  more  mathe- 
matical and  observational  knowledge  than  we  at  present 
possess  to  pronounce  a  final  decision." 

However,  since  La  Place  advanced  his  celebrated 
nebular  hypothesis,  great  advances  in  astronomy  have 
been  made,  and  man  is  in  a  better  position  to  theorize 
on  this  fascinating  problem  today  than  he  was  one  hun- 
dred and  twenty-five  years  ago. 

All   such   theories   must    necessarily   be    regarded   as 


THE  ORIGIN  OF  THE  EARTH  133 

working  hypotheses  only,  to  be  discarded  or  modified 
as  our  knowledge  and  understanding  of  the  laws  of  the 
universe  increase.  No  theory  can  ever  be  regarded  as 
final  or  perfect. 

The  discovery  of  radio-activity  furnishes  us  with 
new  material  for  new  theories.  The  sun  and  the  planets 
may  be  and  probably  are  far  older  than  we  ever  dreamed 
could  be  possible.  It  is  no  longer  necessary  or  reason- 
able to  assume  that  a  greatly  extended  solar  nebula  once 
existed  and  supplied  the  planets  with  heat  through 
gravitational  contraction  or  to  place  a  time  limit  upon 
the  period  required  for  the  formation  of  the  planets 
and  their  satellites  that  is  not  in  accord  with  the  re- 
quirements of  other  sciences. 

We  know  today  that  there  exist  within  the  sun  pow- 
erful repulsive  forces,  which  even  under  present  condi- 
tions occasionally  eject  gaseous  matter  to  heights  of  five 
hundred  thousand  miles  or  more  with  a  velocity  of 
over  two  hundred  miles  per  second.  Small  changes  in 
the  velocity  of  ejection  produce  great  differences  in  the 
height  of  the  ejected  columns. 

With  an  initial  velocity  of  three  hundred  and  eighty 
miles  per  second,  matter  would  be  thrown  from  the 
solar  surface  to  a  height  of  fifty  million  miles.  Were 
the  velocity  of  ejection  three  hundred  and  eighty-three 
miles  per  second  the  height  of  the  column  would  be 
five  hundred  million  miles,  while  a  further  increase 
in  the  initial  velocity  would  send  matter  away  from 
the  sun,  never  to  return. 

Instead  of  suns  and  solar  systems  evolved  from  nebu- 


134      ASTRONOMY  FOR  YOUNG  FOLKS 

lae  we  are  now  more  familiar  with  the  idea  of  nebulae 
evolved  from  stars  through  some  terrific  cataclysm  as 
in  the  case  of  novas  or  temporary  stars. 

It  is  now  known  that  there  exist  in  certain  parts 
of  space  a  number  of  sharply  defined  stars  surrounded 
by  extensive  nebulous  envelopes.  Are  these  possibly 
suns  that  are  going  through  the  process  of  forming 
their  planetary  systems? 

It  is  now  known  that  pressure  of  light  and  electrical 
repulsion  are  forces  to  be  reckoned  with  in  the  evolution 
of  stars  and  nebulae  as  well  as  gravitational  contraction. 
It  has  long  been  felt  that  the  peculiar  formations  exist- 
ing among  the  vast  irregular  gaseous  nebulae  could  not 
be  explained  as  gravitational  effects  alone. 

Light-pressure  and  electrical  repulsion,  as  well  as 
gravitation  are  at  work  within  the  solar  system  and 
the  sun  is  the  seat  of  powerful  disturbances  which  pro- 
duce periodic  outbursts  of  exceptional  activity  and  which 
may  have  produced  in  the  distant  past  more  startling 
effects  than  any  with  which  we  are  familiar  at  present. 

The  earth  and  moon  form  a  system  that  is  in  a  way 
unique.  No  satellite  in  the  solar  system  is  so  large 
in  proportion  to  its  primary  as  is  our  own  moon. 
Seen  from  the  distance  of  Venus  or  Mars,  the  two 
bodies  would  apparently  form  a  double  star.  The  diam- 
eter of  the  moon  is  one- fourth  that  of  the  earth.  Satel- 
lite III  of  Jupiter  far  exceeds  our  own  moon  in  actual 
size  but  its  diameter  is  only  about  four-hundredths 
of  the  diameter  of  the  planet  about  which  it  revolves. 
The  diameter  of  Titan,  the  largest  satellite  of  the  Satur- 


THE  ORIGIN  OF  THE  EARTH  135 

nian  system,  bears  the  same  ratio  to  the  diameter  of 
Saturn.  Moreover,  all  the  nearer  satellites  of  Jupiter 
and  Saturn  lie  nearly  in  the  equatorial  planes  of  these 
planets,  but  the  orbit  of  the  moon  is  inclined  at  a  high 
angle  to  the  plane  of  the  earth's  equator. 

It  is  not  difficult  to  believe  that  the  satellites  of  Jupi- 
ter or  Saturn  were  at  some  time  thrown  off  from  the 
equatorial  belts  of  their  primaries,  just  as  the  planets 
themselves  may  have  been  ejected  from  the  equatorial 
belt  of  the  sun,  but  we  cannot  so  readily  believe  that 
our  own  satellite  was  formed  from  the  earth  in  a  simi- 
lar manner. 

The  moon's  orbit  lies  nearly  in  the  plane  of  the  sun's 
equator,  however,  and  it  is  conceivable  that  both  earth 
and  moon  were  simultaneously  ejected  from  the  equa- 
torial zone  of  the  sun,  the  two  nuclei  being  so  close 
together  that  the  smaller  one  remained  under  the 
gravitational  control  of  the  larger. 

The  difficulties  in  the  way  of  believing  that  the  moon 
once  formed  a  part  of  the  earth  are  very  great.  It 
can  be  shown  mathematically  that  if  the  two  bodies 
at  one  time  formed  a  single  mass  it  would  have  been 
impossible  for  the  moon  to  break  away  from  the  earth, 
unless  the  force  that  caused  the  separation  were  suffi- 
cient to  hurl  the  moon  to  a  greater  distance  than  two 
and  a  half  times  the  earth's  radius.  The  mathema- 
tician, Roche,  found  out  by  computation  that  a  satel- 
lite could  not  remain  intact  within  this  distance  of  the 
planet,  but  would  be  broken  up  into  small  fragments 
under  the  effects  of  the  tides  raised  by  the  larger  body. 


136      ASTRONOMY  FOR  YOUNG  FOLKS 

If,  then,  the  moon  had  originally  been  ejected  from  the 
earth  to  a  less  distance  than  two  and  one-half  radii  of 
the  earth  (2.44  to  be  exact)  it  would  have  been  disinte- 
grated into  small  particles,  or  moonlets,  under  the  tidal 
strains  exerted  upon  it  by  the  earth  and  would  have 
been  gradually  distributed  about  the  earth  in  the  form 
of  a  meteoric  ring  which,  in  the  course  of  ages,  would 
be  absorbed  by  the  earth,  just  as  Saturn  is  now  gradu- 
ally absorbing  its  rings. 

The  planets  differ  greatly  in  density.  The  more  dis- 
tant and  larger  planets — Jupiter,  Saturn,  Uranus  and 
Neptune — have  densities  equal  to  or  less  than  that  of  the 
sun.  The  densities  of  the  inner  planets — Mercury, 
Venus,  Earth  and  Mars — are,  relatively,  extremely  high, 
the  density  of  the  Earth's  core  being  about  that  of 
meteoric  iron.  The  densities  of  Mercury  and  Venus 
are  slightly  less  than  that  of  the  earth  and  the  densities 
of  Mars  and  the  moon  about  equal  to  that  of  the  earth's 
crust. 

If  a  stream  of  matter  were  ejected  from  the  sun  un- 
der the  influence  of  some  external  force,  such  as  that 
exerted  by  a  passing  star,  the  outlying  parts  of  the 
stream  would  consist  of  the  lighter  elements  and  the 
lower  parts  of  the  heavier  elements,  since  the  lighter 
solar  elements  lie  at  or  near  the  surface  of  the  sun  and 
the  heavier  elements  at  greater  depths.  At  the  time  of 
ejection  the  lighter  elements  would  be  thrown  to  great 
distances  and  would  go  to  form  the  less  dense  outer 
planets;  the  heavier  elements  would  go  to  form  the  in- 
ner planets  of  high  density, 


THE  ORIGIN  OF  THE  EARTH  137 

It  is  conceivable  that  ejection  of  solar  material  might 
have  taken  place  under  the  influence  of  certain  forces 
at  work  within  the  sun  itself,  such  as  electrical  repul- 
sion or  pressure  of  light  which  might  become  powerful 
enough  under  certain  conditions  to  overcome  the  effect 
of  gravitation. 

Next  to  nothing  is  known  about  the  physical  state 
of  matter  at  great  solar  depths,  where  abnormal  condi- 
tions of  temperature  and  pressure  must  exist,  and  where 
great  physical  changes  and  disturbances  may  have  taken 
place  in  the  past.  Even  today  solar  activity  goes 
through  a  cycle  of  change  during  the  sun-spot  period, 
and  many  millions  of  years  ago  the  sun-spot  cycle  of 
solar  activity  may  have  been  far  different  from  what 
it  is  today  and  a  far  more  powerful  factor  in  producing 
changes  in  the  solar  system. 

Outbursts  of  novas  indicate  that  agencies  making  for 
peace  and  order  are  not  the  only  ones  at  work  among 
the  stars.  The  cause  of  such  outbursts  has  never  been 
satisfactorily  explained.  The  theory  that  they  are 
caused  by  the  close  approach  of  two  suns  or  by  the 
encounter  of  a  star  with  a  dark  nebula  does  not  ac- 
count for  all  of  the  circumstances  of  such  outbursts. 
The  nebulous  matter  seen  about  a  nova  after  the  out- 
burst is  now  generally  believed  to  have  been  expelled 
from  the  star  itself  at  the  time  of  the  catastrophe  and 
may  conceivably  be  the  stuff  of  which  planetary  sys- 
tems are  made. 

At  some  epoch  in  the  past,  probably  at  least  one 
thousand  million  years  ago,  our  own  sun  may  have 


138      ASTRONOMY  FOR  YOUNG  FOLKS 

undergone  some  cataclysmic  change  and  this  may, 
conceivably,  have  been  brought  about  by  disturbances 
within  the  sun  itself.  Elements  may  have  been  so 
formed  and  distributed  within  the  interior  of  the  sun 
that  friction  and  internal  instability  resulted  and  in 
time  produced  an  upheaval  of  solar  elements  with 
initial  velocities  so  great  that,  possibly,  through  elec- 
trical repulsion*  and  light-pressure,  portions  of  the 
ejected  streams  were  permanently  detached  from  the 
sun  and  became  the  nuclei  of  future  planets.  In  some 
such  way,  it  is  conceivable,  our  own  planet  Earth  and  the 
other  members  of  our  solar  system  may  have  been 
brought  into  existence  in  the  dim  and  distant  past — 
many  hundred  million  years  ago. 


XIX 

JUPITER  AND   HIS  NINE  MOONS 

JUPITER  shines  by  reflected  sunlight  with  a  bril- 
liancy that  usually  exceeds  that  of  the  brightest 
of  the  stars,  Sirius.  When  seen  during  the  midnight 
hours  the  remarkable  unflickering  brightness  of  this 
largest  and  most  distinguished  member  of  the  solar 
system  at  once  serves  to  set  it  apart  from  the  scintilat- 
ing  stars  far  beyond. 

There  is  but  one  planet,  Venus,  that  always  sur- 
passes Jupiter  in  brilliancy,  though  Mars  on  the 
occasions  of  its  close  approaches  to  the  earth  may 
equal  or  slightly  surpass  Jupiter  in  brightness.  As 
Venus  never  departs  more  than  forty-eight  degrees 
from  the  sun,  and  so  is  never  seen  in  the  midnight 
hours,  Jupiter  usually  shines  without  a  rival  when  visible 
at  midnight.  To  one  who  has  observed  the  two  planets 
together  the  silvery  radiance  and  surpassing  brilliancy 
of  Venus,  due  not  to  its  size,  but  to  its  comparative 
nearness  to  the  earth,  at  once  serves  to  distinguish  it 
from  the  golden  glow  of  Jupiter. 

Even  the  smallest  telescopes  of  two-  or  three-inch 
aperture  will  show  the  four  historic  moons  of  Jupiter 
which  were  the  first  celestial  objects  to  be  discovered 
when  Galileo  turned  his  crude  telescope  to  the  heavens 
in  the  year  1610. 

139 


140      ASTRONOMY  FOR  YOUNG  FOLKS 

The  fact  that  these  tiny  points  of  light  were  actually 
revolving  around  the  great  planet  was  soon  detected 
by  the  famous  astronomer  and  we  can  imagine  with 
what  breathless  interest  he  observed  these  satellites 
of  another  world  whose  discovery  dealt  such  a  severe 
blow  to  the  old  Ptolemaic  theory  that  the  earth  was 
the  center  of  the  universe.  It  was  not  until  the 
great  telescopes  of  modern  times  were  invented  that 
the  five  additional  moons  of  Jupiter  were  discovered. 
The  four  satellites  first  observed  by  Galileo  were  fanci- 
fully named  lo,  Europa,  Ganymede  and  Callisto,  in 
the  order  of  their  positions  outward  from  the  planet, 
but  these  names  are  rarely  used  now,  the  satellites 
being  designated  for  convenience  I,  II,  III  and  IV, 
respectively.  The  first  of  the  new  satellites  to  be 
discovered  was  Satellite  V,  which  is  the  nearest  to  Jupi- 
ter of  all  the  nine  moons.  It  is  an  extremely  small 
body,  not  more  than  one  hundred  miles  in  diameter,  and 
to  discover  this  tiny  body  as  it  skirted  rapidly  around 
the  great  planet  within  sixty-seven  thousand  miles  of 
its  surface,  nearly  lost  in  the  glaring  rays,  was  a  diffi- 
cult feat  even  for  an  experienced  observer.  It  was 
accomplished,  however,  by  Prof.  E.  E.  Barnard  with 
the  great  Lick  refractor  in  1892.  Satellite  V  is 
hopelessly  beyond  the  reach  of  any  but  the  greatest 
telescopes,  as  are  also  the  four  satellites  discovered 
since  that  date.  In  fact,  most  of  these  tiny  moons 
are  observed  photographically.  Satellites  VI  and  VII 
were  discovered  photographically  in  1905.  They  are 
both  about  seven  million  miles  from  the  planet  and 


JUPITER  AND  HIS  NINE  MOONS        141 

their  paths  loop  through  one  another;  they  are,  more- 
over, highly  inclined  to  each  other  at  an  angle  of 
nearly  thirty  degrees.  When  nearest  together  they 
are  separated  by  a  distance  of  two  million  miles.  Two 
more  extremely  small  bodies,  known  as  Satellites  VIII 
and  IX,  have  been  discovered  since  then,  one  at 
Greenwich,  England,  in  1908,  the  other  at  the  Lick 
Observatory  in  1914.  These  excessively  faint  bodies 
are  the  most  remote  satellites  of  Jupiter  and  they  are 
of  particular  interest  because  they  travel  around  the 
planet  in  a  retrograde  direction,  or  from  east  to  west, 
which  is  opposite  to  the  direction  of  revolution  pre- 
vailing in  the  solar  system.  The  ninth  and  most  distant 
satellite  of  Saturn  also  retrogrades,  or  revolves  in  a 
clockwise  rather  than  a  counter-clockwise  direction 
around  the  planet.  One  explanation  given  for  this 
peculiarity  of  the  outermost  satellites  of  Jupiter  and 
Saturn  is  that  this  backward  revolution  around  the 
planet  is  more  stable  when  the  satellites  are  at  great 
distances  from  the  primary,  and  the  gravitational  con- 
trol that  the  planet  exerts  is  therefore  weak.  The 
moons  of  the  planets  are,  of  course,  subject  to  the 
attraction  of  the  sun  as  well  as  to  the  attraction  of 
the  controlling  planet,  and  the  greater  the  distance  of 
the  satellite  from  the  planet  the  stronger  the  pull 
exerted  by  the  sun  and  the  weaker  the  bonds  that  bind 
the  moon  to  the  planet.  Beyond  a  certain  limit 
it  would  be  impossible  for  the  planet  to  hold  the  satel- 
lite against  the  sun's  greater  attraction  and  the  satel- 
lite would  leave  the  planet  to  revolve  directly  around 


142   ASTRONOMY  FOR  YOUNG  FOLKS 

the  sun,  thereby  becoming  a  planet.  It  appears  that 
as  this  danger  limit  is  neared  it  is  safer  for  the  satel- 
lite to  "back"  around  the  planet  than  to  follow  the 
usual  "west  to  east"  direction  of  revolution.  The 
eighth  satellite  of  Jupiter  is  more  than  fourteen  mil- 
lion and  the  ninth  more  than  fifteen  million  miles 
from  the  parent  planet  and  they  require  about  two 
years  and  three  years,  respectively,  to  complete  one 
trip  around  Jupiter.  When  we  consider  that  Satellite 
V  darts  around  the  planet  in  less  than  twelve  hours  at 
a  distance  of  only  sixty-seven  thousand  miles  from 
its  surface  we  realise  what  tremendous  differences  exist 
in  the  distances  and  periods  of  revolution  of  the  nine 
moons.  There  is  also  great  disparity  in  the  sizes  of 
the  various  moons.  The  five  moons  discovered  in 
modern  times  are  all  excessively  faint  and  extreme- 
ly small.  The  diameter  of  the  largest  of  these, 
Satellite  V,  is  less  than  one  hundred  miles.  On  the 
other  hand,  the  four  historic  moons  of  Jupiter  are  of 
planetary  dimensions.  The  smallest,  Satellite  II,  is 
slightly  larger  than  our  own  moon,  while  the  largest, 
Satellite  III,  has  a  diameter,  according  to  measurements 
made  with  the  40-inch  Yerkes  refractor  in  1916,  of 
three  thousand  nine  hundred  and  eight  miles,  which 
is  only  four  hundred  miles  less  than  the  diameter  of 
Mars.  The  periods  of  revolution  of  these  four  satel- 
lites range  from  one  day  and  eighteen  hours  for  the 
nearest,  which  is  about  two  hundred  and  sixty-one 
thousand  miles  from  the  center  of  Jupiter,  to  sixteen 
days  and  sixteen  and  one-half  hours  for  the  most  dis- 


JUPITER  AND  HIS  NINE  MOONS        143 

tantf  which  is  more  than  one  million  one  hundred  and 
sixty  thousand  miles  from  the  planet.  These  four 
moons  are  so  near  to  the  great  planet  that  they  are 
continually  dipping  into  his  huge  shadow  and  experi- 
encing an  eclipse  of  the  sun  which,  owing  to  the  near- 
ness and  great  size  of  Jupiter,  lasts  for  two  or  three 
hours.  At  times  of  eclipse  the  moon  suddenly  dis- 
appears from  the  observer's  view,  though  it  may  be 
considerably  to  one  side  of  the  planet.  Its  reappear- 
ance later  on  is  just  as  sudden,  or  it  may  pass  out  of 
the  shadow  while  hidden  from  us  behind  the  disk  of 
the  planet,  in  which  case  its  reappearance  is  invisible 
from  the  earth.  The  occultations  of  the  satellites,  or, 
in  other  words,  their  disappearance  behind  the  planet's 
disk,  are  also  interesting  phenomena  to  observe,  as 
are  their  "transits"  across  the  disk  of  the  planet  as 
the  satellite  passes  in  front  of  it.  Not  only  the 
satellite  itself  but  its  shadow  as  well  can  be  seen, 
a  small  black  dot  passing  over  the  surface  of  Jupiter. 
The  satellite  is  totally  eclipsing  the  sun  for  this 
small  dark  portion  of  the  planet's  disk.  Two 
satellites  and  their  shadows  are  frequently  seen  cross- 
ing the  face  of  the  planet  at  the  same  time.  It 
is  possible  to  observe  all  the  phenomena  of  the  satel- 
lite's transits  and  shadows,  eclipses  and  occultations 
with  very  small  telescopes.  From  observations  of  the 
eclipses  of  Jupiter's  satellites  the  important  discovery 
of  the  finite  velocity  of  light  was  first  made  as  far 
back  as  the  year  1675. 

Faint    surface    markings    have    been    made    out    at 


144   ASTRONOMY  FOR  YOUNG  FOLKS 

certain  times  on  the  largest  of  the  four  satellites, 
Satellite  III,  and  also  on  Satellite  I.  Observations  of 
the  markings  on  the  former  seem  to  indicate  that  it 
always  keeps  the  same  face  turned  toward  Jupiter  as 
does  our  own  moon  toward  the  earth. 

There  are  also  reasons  for  believing  that  the  equa- 
torial regions  of  Satellite  I  are  light  colored  and  the 
polar  regions  dark.  There  is  the  possibility  that  forms 
of  life  may  exist  on  these  satellites  of  Jupiter,  though 
they  are  more  likely  barren,  lifeless  worlds,  such  as 
Mercury  and  the  moon.  Their  great  distance  from  the 
earth,  never  less  than  three  hundred  and  sixty-eight 
million  miles,  makes  observations  of  their  surface 
markings  very  difficult. 

How  beautiful  beyond  description  must  the  heavens 
appear  as  viewed  from  the  satellites  of  Jupiter! 
Viewed  from  the  distance  of  lo,  or  Satellite  I,  the  mighty 
planet  Jupiter  presents  a  spectacle  such  as  the  eye  of 
man  has  never  been  privileged  to  behold.  The  huge 
flattened  globe,  ninety  thousand  miles  in  equatorial 
diameter,  equal  in  mass  to  three  hundred  planets  such 
as  our  own  and  in  volume  to  nearly  fourteen  hundred, 
fills  a  space  in  the  heavens  nearly  twenty  degrees  in 
extent  as  viewed  from  this  satellite.  Fifteen  hundred 
of  our  own  full  moons  would  hardly  fill  the  same 
space.  Whirling  on  its  axis  with  frightful  speed  in 
a  period  of  less  than  ten  hours,  the  huge  ball  glides 
rapidly  but  majestically  onward  through  the  sky.  A 
far  distant  sun  shrunk  to  but  one-fifth  the  diameter 
of  the  full  moon  throws  light  and  shade  across  the 


JUPITER  AND  HIS  NINE  MOONS        145 

rapidly  changing  surface  of  the  planet,  rich  in  the  reds, 
browns  and  yellows  and  all  the  gorgeous  shades  and 
tints  of  its  dense,  seething,  gaseous  envelope.  The 
phases  of  the  moon  on  a  greatly  enlarged  scale  rapidly 
succeed  each  other  on  Jupiter  as  it  is  viewed  from  the 
satellite  in  all  positions  with  reference  to  the  sun.  The 
cause  of  the  belts  of  Jupiter,  that  lie  parallel  to  the  plan- 
et's equator  and  are  constantly  changing  in  number,  width 
and  shade,  as  well  as  the  nature  of  all  the  peculiar 
splashes  of  color  and  intensely  white  flecks  that  come 
and  go  in  the  dense  atmosphere  of  the  planet  would 
not  be  such  a  mystery  to  us  were  it  possible  to  view 
the  great  planet  from  the  distance  of  Satellite  I,  which  is 
about  as  far  from  the  surface  of  Jupiter  as  the  moon 
is  from  the  earth.  It  is  uncertain  whether  the  planet 
is  entirely  gaseous  throughout  or  has  a  central  core 
of  solid  or  liquid  matter.  Its  density  is  only  one  and 
one-quarter  times  that  of  water  and  slightly  less  than  that 
of  the  sun,  showing  that  it  is  composed  largely,  if 
not  entirely,  of  matter  in  a  gaseous  state.  Jupiter  is 
a  world  as  different  from  our  own  as  it  is  possible  to 
imagine.  There  is  no  visible  surface  crust  and  there 
are  no  permanent  markings.  Different  spots  on  the 
planet's  disk  give  different  periods  of  rotation  show- 
ing that  it  is  atmospheric  phenomena  that  we  observe. 
All  is  constant  flux  and  change  on  Jupiter.  Dense 
vapors  arise  from  a  highly  heated  interior  and  spread 
out  into  belts  parallel  to  the  equator  in  the  direction 
of  the  planet's  rotation.  From  its  nearest  satellite 
all  the  interesting  changes  of  color  and  form  that  con- 


146      ASTRONOMY  FOR  YOUNG  FOLKS 

stantly  take  place  in  the  atmosphere  of  this  great  globe 
could  be  observed  in  great  detail.  The  high  percentage 
of  light  and  heat  that  Jupiter  reflects  from  the  sun 
to  its  nearer  satellites  makes  it  a  secondary  sun  to 
them  of  tremendous  size  though  feeble  strength. 

As  seen  from  Satellite  I  the  other  three  major 
moons  of  Jupiter  present  all  the  phases  of  our  own 
moon  in  rapid  succession,  due  to  their  constantly 
changing  positions  with  reference  to  the  sun.  The 
five  small  moons,  discovered  in  modern  times,  are  so 
minute  that  they  are  simply  star-like  points  of  light 
even  when  viewed  from  the  other  moons  of  Jupiter. 

To  keep  track  of  the  rapidly  changing  positions  and 
various  phases  of  the  moons  of  Jupiter  as  seen  from 
any  one  of  them,  as  well  as  the  rapid  apparent  motion 
of  the  planet  through  the  sky  due  to  the  revolutions 
of  the  satellite  around  the  planet,  would  be  a  trouble- 
some task  for  an  astronomer  stationed  on  one  of  these 
far  distant  worlds.  It  would  be  a  common  sight  to 
see  in  the  sky  at  one  time  the  huge  planet,  the  far- 
distant,  shrunken  sun,  and  one,  two  or  three  moons. 
Seen  from  the  moons  of  Jupiter  the  constellations 
would  appear  as  they  do  to  us  on  earth,  for  such  a 
slight  change  in  position  as  five  hundred  million  miles, 
more  or  less,  is  trivial  when  one  is  looking  at  the 
stars.  Observations  of  the  stars  from  the  nearest 
moon  of  Jupiter  would  be  attended  with  great  diffi- 
culties at  times,  since  reflected  sunlight  from  a  body 
nearly  twenty  degrees  in  diameter  would  be  extremely 
troublesome,  especially  were  the  phases  of  the  planets 


JUPITER  AND  HIS  NINE  MOONS       147 

near  that  of  the  full  moon.  We  know  how  the  pres- 
ence of  our  own  moon  in  the  heavens  at  the  full 
dims  the  brightness  of  the  stars  so  that  only  the 
brightest  stars  are  seen.  Even  as  viewed  from  the 
fourth  or  most  distant  of  the  major  satellites  the 
planet  subtends  an  angle  of  nearly  five  degrees.  Occul- 
tations  of  the  stars  are  many  and  frequent  as  the  huge 
planet  globe  glides  swiftly  through  the  heavens.  Many 
a  moonlight  night  appears  almost  as  day  owing  to  the 
presence  of  the  enormous,  brilliantly  reflecting  ball  of 
light  and  at  times  two  or  three  moons  in  addition.  Only 
the  brightest  stars  could  possibly  be  seen  under  such 
circumstances.  When,  however,  the  small  worlds  pass 
into  the  shadow  of  the  great  mother  planet  and  not 
only  the  light  of  the  sun  but  also  the  reflected  light 
of  Jupiter  disappears  for  many  minutes,  the  stars 
shine  forth  in  all  their  glory  there  as  here.  At  such 
times  some  of  the  larger  moons  would  usually  be  seen 
shining  by  the  reflected  light  of  the  far  distant  sun. 
Saturn  also  would  be  visible  as  a  magnificent  star,  but 
beautiful  Venus  and  ruddy  Mars  would  fail  to  appear. 
Tiny  bodies,  mere  specks  of  light  at  this  distance,  they 
would  be  lost  to  view  in  the  glare  of  the  sun. 


XX 

THE   RINGS  AND   MOONS   OF   SATURN 

NEARLY  everyone  has  felt  at  some  time  or  other 
a  strong  desire  to  gaze  at  some  of  the  beauties 
and  wonders  of  the  heavens  through  a  telescope  and 
the  one  object  that  all  of  us  wish  to  see,  if,  perchance, 
this  desire  is  to  be  gratified  is  Saturn,  whose  unusual 
ring  system  has,  so  far  as  we  know,  no  counterpart 
in  the  sky. 

All  the  planets  in  the  solar  system  with  the  excep- 
tion of  the  two  innermost.  Mercury  and  Venus,  are 
attended  by  satellites,  but  Saturn,  alone,  has  in  addition 
to  a  family  of  nine  moons,  three  distinct  rings 
of  great  dimensions  which  are  composed  of  swarms 
of  minute  particles  revolving  around  the  planet. 

Why  Saturn  should  be  the  only  planet  to  possess 
such  a  system  of  rings  has  never  been  explained  in  an 
entirely  satisfactory  manner.  There  is  an  interesting 
law  known  as  "Roche's  Law,"  however,  named  from 
its  investigator,  that  states  that  no  satellite  of  a  planet 
can  exist  intact  wiith  2.44  times  the  radius  of 
the  planet.  This  limit  is  spoken  of  as  "Roche's 
Limit"  and  applying  it  to  the  planet  Saturn  we  find 
that  the  rings  of  Saturn  fall  within  this  limit.  It  does 
not  necessarily  follow  from  this  that  the  minute  parti- 

148 


THE  RINGS  AND  MOONS  OF  SATURN     149 

cles  of  which  the  rings  are  composed  are  the  shat- 
tered remains  of  one  small  satellite,  but  rather  that 
they  are  the  material  from  which  a  satellite  might 
have  been  formed  were  it  not  so  close  to  the  planet. 
Within  "Roche's  Limit"  the  mutual  attraction  of  the 
various  particles  for  each  other  that  would  tend  eventu- 
ally to  gather  them  into  one  body  is  overcome  by  tidal 
forces  that  arise  from  such  close  proximity  to  the 
huge  planet.  The  stress  and  strain  of  such  forces  is 
so  great  that  no  grouping  of  particles  can  take  place. 
This  explains,  possibly,  why  the  rings  continue  to 
exist  in  their  present  condition.  The  total  quantity  of 
matter  in  the  rings  is  known  to  be  very  small,  for 
it  does  not  disturb  the  motions  of  any  of  the  nearer 
and  smaller  satellites,  though  tiny  Mimas,  six  hundred 
miles  in  diameter,  is  only  thirty-one  thousand  miles 
beyond  the  outer  edge  of  the  outer  ring. 

An  interesting  observation  was  made  a  few  years 
ago  of  the  passage  of  the  rings  of  the  planet  between 
us  and  a  star.  Though  the  light  of  the  star  was 
diminished  to  one- fourth  of  its  normal  brightness 
when  the  rings  passed  before  it,  at  no  time  was  its 
light  entirely  eclipsed  by  any  of  the  particles.  It  was 
computed  that  if  the  diameters  of  any  of  the  individual 
particles  had  amounted  to  as  much  as  three  or  four 
miles  the  star  would  have  been  temporarily  eclipsed. 
An  upper  limit  for  the  size  of  the  moonlets  was  thus 
obtained.  The  average  diameter  of  the  particles  is 
probably  much  less  than  three  miles. 

The  thickness  of  the  ring  system  is  not  over  fifty 


150      ASTRONOMY  FOR  YOUNG  FOLKS 

or  one  hundred  miles,  but  its  total  diameter  is  one 
hundred  and  seventy-two  thousand  miles.  There  are, 
in  all,  three  concentric  rings.  The  faint  inner  ring, 
known  as  the  "crape"  ring,  is  invisible  in  a  telescope 
under  four  inches  in  aperture.  The  width  of  this 
inner  ring  is  eleven  thousand  miles.  Just  beyond  the 
crape  ring  is  the  chief,  bright  ring,  eighteen  thousand 
miles  in  width.  It  shades  gradually  in  brightness  from 
its  juncture  with  the  crape  ring  to  its  most  luminous 
portion  at  its  outer  edge,  which  is  separated  from 
the  third  or  outer  ring  by  a  gap  two  thousand  two 
hundred  miles  in  width,  known  as  Cassini's  Division. 
The  third  or  outer  ring  is  eleven  thousand  miles  wide 
and  is  less  bright  than  the  central  ring.  The  inner  edge 
of  the  inner  ring  is  but  six  thousand  miles  above 
the  surface  of  the  planet.  On  account  of  the  curvature 
of  the  planet  the  ring  system  is  invisible  from  the 
north  and  south  poles  of  Saturn.  As  in  the  case  of 
the  satellites  of  a  planet  the  inner  particles  of  the 
rings  revolve  around  the  planet  more  rapidly  than  the 
outer  particles.  The  innermost  particles  of  the  crape 
ring  require  but  five  hours  for  one  journey  around 
Saturn  while  the  outermost  particles  of  the  outer 
ring  require  one  hundred  and  thirty-seven  hours,  or 
nearly  six  days  to  complete  one  revolution. 

In  addition  to  the  gap  in  the  rings  known  as  Cas- 
sini's  Division  several  other  fainter  divisions  exist. 
If  a  group  of  moonlets  were  to  revolve  around  the 
planet  in  the  positions  marked  by  these  gaps  their 
periods  of  revolution  would  be  commensurable  with 


THE  RINGS  AND  MOONS  OF  SATURN    151 

the  periods  of  several  of  the  satellites  of  Saturn.  As  a 
result  the  attraction  exerted  on  such  particles  by  these 
satellites  would  gradually  disturb  their  motion  in  such 
a  way  as  to  draw  them  away  from  these  positions. 
It  is  owing,  therefore,  to  the  attraction  of  the  satel- 
lites of  Saturn  for  the  moonlets  that  these  gaps  in 
the  rings  exist. 

As  a  result  of  the  disturbances  produced  in  the 
motion  of  the  moonlets  by  the  satellites  of  Saturn 
collisions  are  bound  to  occur  occasionally  among  the 
various  particles.  When  two  particles  collide  the 
period  of  revolution  of  one  or  both  of  them  is  reduced 
and  as  a  result  collisions  tend  to  bring  the  moonlets 
gradually  closer  and  closer  to  the  surface  of  the 
planet.  The  dusky  inner  ring,  it  is  believed,  may  con- 
sist largely  of  particles  whose  periods  have  been  con- 
tinually shortened  by  collisions. 

Saturn  may,  therefore,  lose  its  ring  system  in  the 
course  of  time  through  its  gradually  being  drawn  down 
upon  the  planet  by  collisions  of  the  various  particles 
until  all  of  the  material  is  finally  swept  up  by  the  planet. 
Such  a  change  would  probably  require  millions  of 
years,  however,  as  collisions  are  probably,  on  the 
whole,  infrequent.  It  is  possible  that  the  ring  system 
of  Saturn  may  have  been  much  more  extensive  in 
the  past  than  it  is  now  and  other  members  of  our 
solar  system  may  have  had  such  appendages  in  the 
far  distant  past. 

The  appearance  of  the  rings  of  Saturn  as  viewed 
from  our  planet  changes  periodically  as  a  result  of 


152      ASTRONOMY  FOR  YOUNG  FOLKS 

the  revolution  of  the  earth  and  Saturn  around  the 
sun,  which  places  them  in  constantly  changing  positions 
with  reference  to  each  other.  The  rings  He  in  the 
plane  of  Saturn's  equator,  which  is  inclined  twenty- 
seven  degrees  to  its  orbit  and  twenty-eight  degrees  to 
the  Earth's  orbit. 

Since  the  position  of  the  equator  remains  parallel 
to  itself  while  the  planet  is  journeying  around  the  sun 
it  happens  that  half  the  time  the  earth  is  elevated 
above  the  plane  of  the  rings  and  the  remainder  of  the 
time  it  lies  below  the  plane  of  the  rings.  Twice  in 
the  period  of  Saturn's  revolution  around  the  sun, 
which  occupies  nearly  thirty  years,  the  earth  lies 
directly  in  the  plane  of  the  rings  and  at  this  time  the 
rings  entirely  disappear  from  view  for  a  short  time. 
Mid-way  between  the  two  dates  of  disappearance  the 
rings  are  tilted  at  their  widest  angle  with  reference 
to  the  earth  and  they  are  then  seen  to  the  best  ad- 
vantage. As  the  date  of  their  disappearance  approaches 
they  appear  more  and  more  like  a  line  of  light  ex- 
tending to  either  side  of  the  planet's  equator.  Even 
in  the  most  powerful  telescope  the  rings  entirely  disap- 
pear from  view  for  a  few  hours  at  the  time  the  earth 
lies  exactly  in  the  same  plane.  It  is  at  this  time  that 
the  ball  of  the  planet  is  best  seen.  Its  flattening  at 
the  poles,  which  is  nearly  ten  per  cent,  of  its  equa- 
torial diameter  then  gives  it  a  decidedly  oval  appear- 
ance. Ordinarily  one  of  the  hemispheres  of  Saturn  is 
partly  or  entirely  concealed  by  the  rings  so  that  the 
oblate  form  is  not  so  noticeable.  It  was  the  change 


THE  RINGS  AND  MOONS  OF  SATURN    153 

in  the  tilt  and  visibility  of  the  rings  that  so  perplexed 
Galileo  when  he  attempted  to  make  out  the  nature 
of  these  appendages  of  Saturn  with  his  crude  telescope 
of  insufficient  magnifying  power.  So  great  was  his 
bewilderment  when  the  rings  finally  disappeared  that 
he  cried  out  in  despair  that  Saturn  must  have  swal- 
lowed his  children,  according  to  the  legend.  He  finally 
became  so  exasperated  with  the  results  of  his  observa- 
tions that  he  gave  up  observing  the  planet.  The  true 
nature  of  these  appendages  of  Saturn  remained  a 
mystery  until  Huygens  solved  the  problem  in  1655, 
some  time  after  the  death  of  Galileo. 

In  addition  to  the  rings,  Saturn  has  nine  satellites 
named,  in  the  order  of  their  distance  outward  from 
the  planet,  Mimas,  Enceladus,  Tethys,  Dione,  Rhea, 
Titan,  Hyperion,  lapetus  and  Phoebe.  The  last-men- 
tioned satellite  was  discovered  by  W.  H.  Pickering 
in  1899.  It  aroused  great  interest  at  the  time  because 
it  was  the  first  satellite  to  be  discovered  with  "retro- 
grade" motion  in  its  orbit.  Two  satellites  of  Jupiter 
since  discovered  revolve  in  the  same  direction  around 
their  primary. 

The  satellites  of  Saturn  are  approximate  to  those  of 
Jupiter  in  size  and  exactly  equal  them  in  number. 
The  largest,  Titan,  is  three  thousand  miles  in  diameter 
and  can  be  easily  seen  with  the  smallest  telescopes. 
With  a  four-inch  telescope  five  of  the  satellites  can 
be  readily  found,  though  they  are  not  as  interesting 
to  observe  as  the  satellites  of  Jupiter  because  they 
are  far  more  distant  from  the  earth.  The  time  they  re- 


154      ASTRONOMY  FOR  YOUNG  FOLKS 

quire  to  make  one  journey  around  Saturn  varies  from 
nearly  twenty-three  hours  for  Mimas,  the  nearest,  to 
approximately  five  hundred  and  twenty- four  days  for 
Phoebe,  the  most  distant. 

Saturn  as  well  as  Jupiter  is  marked  by  belts  parallel 
to  the  Equator  though  they  appear  more  indistinct  than 
those  of  Jupiter  on  account  of  the  greater  distance 
of  Saturn.  Saturn  also  resembles  Jupiter  in  its  physical 
composition  which  is  largely,  if  not  entirely,  gaseous, 
and  in  the  extremely  short  period  of  rotation  on  its 
axis  which  is  approximately  ten  hours  In  more  ways 
than  one  Saturn  is  a  very  unusual  planet.  In  addi- 
tion to  possessing  an  enormous  ring  system  it  is  the 
lightest  of  all  the  planets,  its  density  being  only  sixty- 
three  hundredths  that  of  water,  and  it  is  the  most 
oblate,  its  flattening  at  the  poles  amounting  nearly  to 
one-tenth  of  its  diameter.  Its  equator  is  more  highly 
inclined  to  its  orbit  than  is  the  case  with  any  other 
planet,  not  even  excepting  the  earth  and  Mars.  For 
this  reason  its  seasonal  changes  are  very  great,  in 
marked  contrast  to  Jupiter  whose  equator  lies  very 
nearly  in  the  plane  of  its  orbit.  Since  Saturn  is  so 
far  away  from  the  sun  that  it  receives  only  one 
ninetieth  as  much  light  and  heat  per  unit  area  as  the 
earth,  its  outer  gaseous  surface  must  be  extremely 
cold  unless  considerable  heat  is  conveyed  to  the  sur- 
face from  within  its  hot  interior. 

The  late  Prof.  Lowell  concluded  from  certain  ob- 
servations made  at  Flagstaff,  Ariz.,  that  Saturn  is 
composed  of  layers  of  different  densities  and  that  the 


THE  RINGS  AND  MOONS  OF  SATURN    155 

inner  layers  are  more  flattened  at  the  poles  and  rotate 
faster  than  the  outer  layers.  Marked  variations  in  the 
color  and  brightness  of  the  ball  of  the  planet  have  been 
noted  from  time  to  time.  In  1916  observers  of  Saturn 
described  the  planet  as  pinkish-brown  and  conspicuously 
darker  than  the  brighter  portions  of  the  rings. 

It  is  believed  that  these  very  noticeable  changes  in 
the  color  and  brightness  of  Saturn  are  due  to  slight, 
irregular  changes  in  the  intensity  of  the  radiations  of 
the  sun  which  set  up  certain  secondary  effects  in  the 
atmosphere  of  the  planets.  Similar  changes  in  color 
and  brightness  have  been  observed  also  in  the  case 
of  Jupiter. 


XXI 

IS  THE  MOON  A  DEAD  WORLD? 

IT  has  been  a  generally  accepted  belief  among 
astronomers  for  years  that  the  moon  is  a  dead 
world  devoid  of  air  and  water  and  so,  necessarily, 
lifeless.  It  is  certain  that  the  moon  has  no  extensive 
atmosphere  such  as  envelops  our  own  planet.  There 
is  abundant  proof  of  this  fact.  The  edge  of  the  lunar 
disk  is  clear-cut.  Whenever,  as  happens  frequently, 
the  moon  passes  between  us  and  a  star  the  disappear- 
ance of  the  star  is  instantaneous.  There  is  no  gradual 
dimming  or  refraction  of  the  star's  light  by  atmospheric 
vapors.  Moreover,  lunar  shadows  are  harsh  and  black. 
There  is  no  evidence  of  diffusion  of  light  on  the  moon 
by  atmospheric  gases. 

The  absence  of  water  or  water  vapor  on  the  visible 
surface  of  the  moon,  at  least  in  any  appreciable  quantity, 
is  plainly  evident  to  anyone  who  observes  the  moon 
through  the  telescope.  Even  with  small  telescopes, 
objects  five  miles  or  so  in  diameter  can  be  readily 
detected  and  clouds  drifting  over  the  surface  could 
not  possibly  escape  our  observation  if  they  existed. 

Bodies  of  water,  great  or  small,  would  be  plainly 
visible  and  would  besides  give  rise  to  water  vapor 
and  clouds,  which  we  would  not  fail  to  detect. 

156 


IS  THE  MOON  A  DEAD  WORLD?        157 

Since  the  surface  of  the  moon  is  unscreened  by 
air  and  water  vapor  to  absorb  the  incoming  rays  from 
the  sun,  and  the  outgoing  radiations  from  the  surface, 
the  extremes  of  temperature  between  day  and  night 
are  very  great,  and  are  augmented  by  the  fact  that 
the  lunar  day  equals  the  lunar  month  in  length,  so 
that  fourteen  days  of  untempered  heat  are  followed  by 
fourteen  days  of  frigid  darkness.  Observations  of  the 
rate  of  radiation  from  the  moon's  surface  during  total 
eclipses  of  the  moon  indicate  that  the  moon's  radiation 
is  very  rapid,  and  that  its  temperature  during  the  height 
of  the  lunar  day  probably  approaches  200°  F.,  while 
at  the  lunar  midnight  it  may  have  fallen  to  100° 
below  zero,  F,  or  even  lower. 

With  air  and  water  both  lacking  and  such  extremes 
of  temperature  existing  why  should  we  seriously  con- 
sider the  question  of  life  on  the  moon? 

This  is  the  point  of  view  of  the  majority  of  astrono- 
mers and  it  seems  well  taken.  Yet  many  astronomers 
who  have  made  a  special  study  of  the  lunar  surface 
for  years  under  all  conditions  of  illumination  and 
phase,  and  have  most  carefully  observed  and  mapped 
and  photographed  its  characteristic  markings,  are 
agreed  that  there  are  evidences  that  changes  are  taking 
place  on  the  moon,  and  recently  Prof.  W.  H. 'Picker- 
ing has  expressed  the  belief,  substantiated  by  draw- 
ings, that  there  is  a  progressive  change  of  color  or 
darkening  within  certain  lunar  craters  with  the  advance 
of  the  lunar  day,  indicating,  in  his  opinion,  a  rapid 
vegetational  growth  that  springs  up  in  the  height  of 


158      ASTRONOMY  FOR  YOUNG  FOLKS 

the  lunar  day  and  dies  out  as  the  lunar  night  approaches. 

Some  years  ago  certain  selenographers  suggested 
that  there  might  exist  in  the  numberless  crater-pits  and 
craters,  in  the  deep-lying  maria  or  "seas,"  and  in  the 
clefts  and  rills  and  cracks  that  form  intricate  systems 
all  over  the  lunar  surface,  certain  exhalations  from  the 
surface  and  heavy  vapors  including  possibly  carbon 
dioxide  and  water  vapor  to  temper  the  extremes  of  the 
long  lunar  days  and  night  and  furnish  the  necessary 
medium  for  the  support  of  certain  forms  of  animal  and 
vegetable  life. 

Many  astronomers,  including  a  number  who  are  not 
in  sympathy  with  the  above  view,  believe  that  snow  and 
ice  exist  on  the  moon,  even  though  water  in  the  form 
of  liquid  and  vapor  is  not  observable.  All  the  extremely 
brilliant  portions  of  the  surface,  according  to  some 
astronomers,  are  covered  with  snow  and  ice.  Certainly, 
some  portions  of  the  moon's  surface  reflect  sunlight  as 
brilliantly  as  if  they  were  covered  with  freshly  fallen 
snow,  while  other  portions  appear  to  be  black  by 
contrast.  There  also  appears  to  be  evidence  that  cer- 
tain small  markings,  described  as  crater-cones  and  re- 
sembling our  terrestrial  volcanoes  more  than  any  other 
lunar  feature,  are  at  times  temporarily  obscured  from 
view  by  a  veil  of  vapors.  Many  observers  believe  that 
these  crater-cones  are  active  volcanic  vents,  and  that 
there  is  considerable  volcanic  activity  still  taking  place 
upon  the  moon. 

These  small  crater-cones  resemble,  we  are  told,  para- 
sitic cones  found  on  the  sides  of  terrestrial  volcanoes, 


IS  THE  MOON  A  DEAD  WORLD?        159 

and  they  are  frequently  seen  on  the  floors  of  craters 
closely  associated  with  light  streaks.  These  crater- 
cones  appear  under  a  high  sun  as  minute  white  spots 
and  can  be  studied  to  advantage  only  with  powerful 
instruments.  The  Italian  astronomer,  Maggini,  observ- 
ing the  floor  of  the  lunar  crater,  Plato,  in  1916  noted 
that  one  of  the  small  crater-cones  that  exist  there  in 
great  numbers,  was  temporarily  obscured  from  view  by 
a  cloud  of  reddish  vapors,  and  Prof.  W.  H.  Pickering, 
at  Arequipa,  Peru,  observing  the  same  region  some  years 
ago,  believed  that  he  saw  evidence  of  change  in  some  of 
these  small  markings.  The  crater,  Plato,  has  probably 
been  more  carefully  studied  than  any  other  portion  of  the 
lunar  surface.  It  is  sixty  miles  in  diameter  and  may  be 
seen  even  without  a  telescope  as  a  dark  "eye"  not  far 
from  the  northern  edge  of  the  moon.  Its  floor  is  one 
of  the  darkest  objects  in  the  moon — a  dark  steel-grey 
in  color — and  there  is  no  doubt  that  for  some  unknown 
reason  its  dark  hue  deepens  from  the  time  the  sun  has 
an  altitude  of  twenty  degrees  until  after  full  moon.  It 
has  a  brilliant  white  wall  rising  from  3,000  to  4,000 
feet  above  its  floor,  crowned  with  several  lofty  peaks 
and  intersected  by  a  number  of  valleys  and  passes. 
The  spots  and  faint  light  markings  on  the  floor  have 
been  the  object  of  much  study  with  small  or  moderate 
sized  instruments,  and  at  least  six  of  them  are  known 
to  be  crater-cones.  Since  they  can  only  be  studied  to 
advantage  with  powerful  instruments  and  as  such  in- 
struments are  rarely  used  for  a  systematic  study  of 
lunar  markings,  it  is  difficult  to  settle  the  controversy 


160      ASTRONOMY  FOR  YOUNG  FOLKS 

as  to  whether  they  have  changed  in  appearance  or  have 
been  at  any  time  obscured  by  vapors.  Most  lunar  ob- 
serving is  done — necessarily — with  smaller  instruments 
because  the  majority  of  astronomers  appear  to  have 
accepted  the  view  that  the  moon  is  a  dead  world,  and 
those  who  are  engaged  in  astronomical  work  with  our 
greatest  telescopes  seem  to  feel  that  other  fields  of 
research  will  prove  more  fruitful.  Possibly  it  is  for 
this  reason  that  we  know  so  little  about  our  nearest 
neighbor  in  space !  There  are  at  least  as  many  unsolved 
problems  confronting  us  on  the  moon  as  there  are 
among  the  distant  stars. 

Geologists  tell  us  that  more  oxygen  is  to  be  found 
in  the  first  six  feet  of  the  earth's  crust  than  in  all 
of  the  atmosphere  above.  Does  oxygen  not  exist  in 
the  surface  rocks  of  the  moon  as  well? 

Volcanic  action,  we  are  told,  is  primarily  an  escape 
of  gases  from  the  interior,  chiefly  hydrogen,  nitrogen, 
hydro-carbons,  sulphur,  and  various  compounds,  as  well 
as  vast  quantities  of  steam.  Beneath  the  surface  chemi- 
cal change  is  continually  taking  place  which  results  in 
the  release  of  an  enormous  amount  of  heat.  Some 
of  the  gases  mentioned  above  combine  with  the  oxygen 
in  the  surface  rocks  and  heat  is  evolved.  It  is  a 
known  fact  that  there  is  great  inherent  heat  in  the 
earth's  surface  crust.  Why  not  in  the  moon's  surface 
crust  as  well? 

The  water  that  would  be  expelled  in  the  form  of 
steam  from  volcanic  vents  on  the  moon  would  be 
transformed  immediately  into  hoar-frost,  snow  and  ice 


IS  THE  MOON  A  DEAD  WORLD?        161 

and  would  settle  down  upon  the  flanks  of  the  crater- 
cones  or  vents. 

It  should  be  borne  in  mind  that  only  volcanic  activity 
on  an  enormous  scale  would  be  plainly  visible  to  us 
even  with  the  powerful  telescopes  at  our  command. 
Ordinary  eruptions  such  as  occur  on  our  own  planet 
would  be  very  difficult  to  detect.  Since  the  escaping 
vapors  would  rapidly  pass  into  the  solid  state  and  settle 
down  upon  the  flanks  of  the  crater-cones  or  vents,  we 
would  observe  in  general  little  if  any  change  in  an 
object  unless  we  chanced  to  be  looking  at  it  at  the 
time  of  the  eruption,  when  it  might  appear  to  be 
temporarily  obscured  by  a  veil  of  vapors.  What  are 
the  chances  that  we  would  be  carefully  observing  at  the 
precise  time  of  an  eruption,  a  minute  marking,  two  or 
three  miles  in  diameter,  on  a  surface  as  large  as  all 
of  North  America,  a  surface  that  is  covered  with  some 
30,000  charted  craters,  numberless  crater-pits,  streaks, 
rays,  spots,  clefts  and  rills  in  intricate  systems,  moun- 
tain chains  and  valleys  and  a  mass  of  intricate  detail? 

If  we  were  looking  at  the  earth  from  the  moon  with 
the  aid  of  a  powerful  telescope  would  we  be  apt  to 
notice  an  eruption  of  Vesuvius  or  Katmai  or  Mauna 
Loa?  Objects  four  or  five  miles  in  diameter  would 
appear  as  hazy  spots  with  nothing  distinctive  or  remark- 
able in  their  appearance.  Yet  vapor  and  steam  arising 
from  terrestrial  volcanoes  would  be  carried  by  our 
atmosphere  over  an  area  of  many  square  miles,  while 
there  is  no  atmosphere  on  the  moon  to  spread  the  vapors 
that  may  arise  from  similar  volcanic  vents.  It  would 


162      ASTRONOMY  FOR  YOUNG  FOLKS 

have  to  be  a  cataclysmic  change  indeed  to  be  accepted 
as  indisputable  evidence  that  change  is  taking  place 
on  the  moon,  and  the  days  of  gigantic  upheavals  are 
probably  over  on  our  satellite  as  well  as  on  the  earth. 
If  volcanic  activity  is  still  taking  place  on  the  moon  it 
is  probably  in  a  mild  form  such  as  a  comparatively 
quiet  emission  of  gases  from  volcanic  vents  and  fuma- 
roles.  Such  forms  of  activity  would  not  be  plainly 
visible  at  this  distance,  even  with  the  aid  of  powerful 
telescopes.  The  problem  of  detecting  changes  on  the 
moon  is  complicated  by  the  fact  that  a  change  of  il- 
lumination greatly  alters  the  appearance  of  all  lunar 
markings.  Such  a  change  is  continually  taking  place 
in  the  course  of  the  month.  A  marking  that  stands  out 
in  bold  relief  at  lunar  sunrise  or  sunset  will  change 
entirely  in  appearance  a  few  days  later  under  a  high 
sun  or  even  disappear  from  view  entirely.  These 
changes  in  phase  or  illumination  have  to  be  taken  ac- 
count of  in  the  search  for  evidence  of  actual  change. 
To  decide  whether  or  not  change  has  actually  taken 
place  the  object  must  be  viewed  under  similar  condi- 
tions, so  far  as  they  can  be  obtained.  Even  when 
special  care  is  taken  in  this  respect  the  suspected  evi- 
dence of  change  is  usually  "explained  away"  as  due 
to  differences  in  illumination  or  seeing,  by  those  who 
have  not  observed  the  object  themselves  and  are  not 
in  sympathy  with  the  view  that  the  moon  is  anything 
but  a  dead  world. 

As  regards  the  question  of  life  on  the  moon,  it  is 
interesting  to  consider  the  facts  brought  out  by  investi- 


IS  THE  MOON  A  DEAD  WORLD?        163 

gations  made  by  scientists  connected  with  the  Geo- 
physical Laboratory  of  the  Carnegie  Institute  in  the 
Valley  of  Ten  Thousand  Smokes.  The  volcanic  activ- 
ity there  takes  the  form  of  eruptions  from  numerous 
small  vents  or  fumaroles  and  ninety-nine  per  cent,  of 
the  emanations  are  water  vapor.  It  was  observed  that 
blue-green  algae  were  living  at  the  edge  of  active  vents 
emitting  ammonia  compounds  at  a  temperature  of 
212°  F.  They  were  not  found,  however,  near  vents 
from  which  ammonia  compounds  were  not  being 
emitted.  If  life  exists  under  such  conditions  it  is  con- 
ceivable that  suitable  conditions  for  the  support  of 
certain  forms  of  life,  animal  as  well  as  vegetable,  may 
be  found  in  low-lying  valleys  and  crevices  and  upon 
the  floors  of  craters,  where  certain  gases  essential  to  the 
support  of  life  might  be  evolved  from  many  small  vol- 
canic vents  and  fumaroles. 

Many  theories  have  been  advanced  to  explain  the 
origin  of  the  lunar  craters  which  have  no  counterpart 
on  our  own  planet.  They  are  saucer-like  depressions 
in  the  surface  of  the  moon,  frequently  of  such  great 
size  that  an  observer  standing  in  the  center  would  not 
be  able  to  see  either  side  of  the  crater  owing  to  the 
curvature  of  the  moon's  surface.  Craters  fifty,  sixty 
or  one  hundred  miles  in  diameter  are  by  no  means 
uncommon,  while  there  are  thousands  between  five  and 
fifty  miles  in  diameter.  A  characteristic  feature  of 
many  craters  is  a  central  peak,  and  the  surrounding 
walls  are  often  a  mile  or  more  high  and  in  some 
instances  are  symmetrically  terraced.  New  craters 
have  been  formed  on  the  sides  or  floors  of  old  craters, 


164      ASTRONOMY  FOR  YOUNG  FOLKS 

and  these  are  always  more  clear-cut  and  sharper  in  out- 
lines than  the  old  formations,  and  generally  much 
smaller.  A  number  of  craters  are  surrounded  by  a  system 
of  light  streaks  or  rays  of  unknown  origin  that  extend 
in  some  instances  to  enormous  distances  on  all  sides  of 
the  crater.  The  most  conspicuous  system  is  the  one 
surrounding  the  lunar  crater  Tycho  near  the  south 
pole  of  the  moon.  The  rays  originating  in  this  crater 
extend  in  all  directions  for  hundreds  of  miles  without 
turning  aside  for  any  obstructions,  passing  over  moun- 
tains, craters  and  plains  in  their  course  in  practically 
straight  lines  like  spokes  in  a  wheel.  This  ray  system 
of  Tycho  is  the  most  noticeable  marking  on  the  moon's 
surface  at  the  time  of  full  moon.  As  these  streaks 
cast  no  shadow  they  are  apparently  cracks  in  the  sur- 
face that  have  rilled  up  with  some  light-colored  ma- 
terial from  below.  Their  origin  has  never  been  satis- 
factorily explained. 

As  to  the  origin  of  the  lunar  craters,  some  believe 
that  they  were  produced  in  past  ages  by  a  bombardment 
of  the  lunar  surface  by  huge  meteoric  masses;  but  there 
are  many  objections  to  this  theory  that  we  will  not 
take  up  here.  It  is  more  generally  believed  that  the 
lunar  craters  are  a  result  of  volcanic  activity  on  an 
enormous  scale  which  took  place  on  the  moon  many 
ages  ago  and  which  has  now  practically  ceased,  its  only 
manifestations  now  taking  the  form  of  a  quiet  emis- 
sion of  gases  from  small  volcanic  vents  or  fumaroles 
which  exist  all  over  the  lunar  surface  but  which  are 
to  be  found  in  greatest  numbers  on  the  floors  and  sides 
of  craters. 


XXII 
COMETS 

THE  orbits  of  comets  are  inclined  at  all  angles  to 
each  other  and  to  the  orbits  of  the  planets  which, 
on  the  other  hand,  lie  very  nearly  in  the  same  plane. 

The  larger  members  of  the  sun's  family,  the  planets 
and  their  satellites,  revolve  from  west  to  east  around 
the  sun.  Comets  on  the  contrary  frequently  retrograde 
or  back  around  the  sun  in  the  opposite  direction — 
from  east  to  west. 

The  paths  that  these  erratic  visitors  follow  in  their 
journeys  around  the  sun  bear  not  the  slightest  resem- 
blance to  the  paths  of  the  planets,  which  are  almost 
perfect  circles.  The  orbits  of  comets  are  ellipses  that 
are  greatly  elongated  or  parabolas.  If  the  orbit  is  a 
parabola  the  comet  makes  one  and  only  one  visit  to 
the  sun,  coming  from  interstellar  space  and  returning 
thereto  after  a  brief  sojourn  within  our  solar  system. 

Donati's  comet  of  1858,  one  of  the  greatest  comets 
of  the  nineteenth  century,  had  a  period  of  more  than 
two  thousand  years  and  its  aphelion  (the  point  in  its 
orbit  farthest  away  from  the  sun)  was  five  times  more 
distant  that  the  orbit  of  Neptune. 

There  is,  however,  a  class  of  comets  known  as 
periodic  comets  that  have  extremely  short  periods  of 

165 


166      ASTRONOMY  FOR  YOUNG  FOLKS 

revolution  around  the  sun.  To  this  class  belongs 
Halley's  comet  whose  period  of  seventy-five  years 
exceeds  that  of  any  other  short  period  comet.  Encke's 
comet,  on  the  other  hand,  has  a  period  of  three  and  a 
third  years  which  is  the  shortest  cometary  period  known. 
Most  of  the  periodic  comets  are  inconspicuous  and 
only  visible  telescopically  even  when  comparatively 
near  to  the  earth.  Halley's  comet  is  the  only  one  of 
this  class  that  lays  any  pretensions  to  remarkable  size 
or  brilliancy  and  it  also  is  showing  the  effects  of  dis- 
integration resulting  from  too  frequent  visits  to  the  sun. 
Comets  are  bodies  of  great  bulk  or  volume  and  small 
total  mass.  Their  tails,  which  only  develop  in  the 
vicinity  of  the  sun,  are  formed  of  the  rarest  gases,  and 
the  best  vacuum  that  man  can  produce  would  not  be 
in  as  tenuous  a  state  as  the  material  existing  in  the  tails 
of  comets.  There  are  many  proofs  of  the  extreme 
tenuity  of  comets.  The  earth  has  on  a  number  of 
occasions  passed  directly  through  the  tails  of  comets 
without  experiencing  the  slightest  visible  effects.  Stars 
shine  undimmed  in  luster  even  through  the  heads  of 
comets.  If  the  earth  should  encounter  a  comet  "head 
on"  it  is  doubtful  if  it  would  experience  anything  more 
serious  than  a  shower  of  meteors  which  would  be  con- 
sumed by  friction  with  the  earth's  atmosphere,  or  a  fall 
of  meteorites  over  a  small  area  of  a  few  square  miles. 
It  is  possible,  however,  that  matter  in  the  nucleus,  the 
star-like  condensation  in  the  head  of  a  comet,  may  con- 
sist of  individual  particles  weighing  in  some  in- 
stances a  number  of  tons,  surrounded  by  a  gaseous 


COMETS  167 

envelope  and  held  together  by  the  loose  bonds  of 
their  mutual  attraction.  If  the  earth  should  encounter 
the  nucleus  of  a  comet  considerable  damage  might  be 
done  over  a  portion  of  the  earth's  surface,  but  the 
chances  of  such  an  occurrence  are  less  than  one  in  a 
million. 

Since  the  total  mass  of  a  comet  is  so  small,  a  close 
approach  to  one  of  the  planets,  especially  Jupiter,  pro- 
duces great  changes  in  the  form  of  the  comet's  orbit, 
though  the  motion  of  the  planet  is  not  disturbed  in  the 
slightest  degree  by  the  encounter. 

The  majority  of  all  the  short-period  comets  have  been 
"captured"  by  Jupiter,  that  is,  the  original  orbits  have 
been  so  changed  by  the  perturbations  produced  by  close 
approaches  to  the  giant  planet  that  their  aphelia,  or  the 
points  in  their  orbits  farthest  from  the  sun,  lie  in  the 
vicinity  of  Jupiter's  orbit.  Several  of  the  other  planets 
have  also  "captured"  comets  in  this  sense,  and  the  fact 
that  the  aphelia  of  a  number  of  comets  are  grouped  at 
certain  definite  intervals  beyond  the  orbit  of  Neptune 
has  been  considered  by  some  astronomers  to  be  an  in- 
dication that  there  are  two  or  more  additional  planets 
in  the  solar  system  revolving  around  the  sun  at  these 
distances. 

The  most  interesting  feature  of  a  comet  is  its  char- 
acteristic tail  which  develops  and  increases  in  size  and 
brilliancy  as  the  comet  approaches  the  sun.  As  the  tail 
is  always  turned  away  from  the  sun  it  follows  the 
comet  as  it  draws  near  the  sun  and  precedes  it  as  it 
departs.  Its  origin  is  due,  it  is  believed,  both  to  elec- 


168   ASTRONOMY  FOR  YOUNG  FOLKS 

trical  repulsion  and  light-pressure  acting  upon  minute 
particles  of  matter  in  the  coma  or  head  of  the  comet. 

The  curvature  of  the  tail  depends  upon  the  nature  of 
the  gases  of  which  it  is  composed.  Long,  straight  tails 
consist  chiefly  of  hydrogen,  it  has  been  found,  curved 
tails  of  hydrocarbons  and  short,  bushy  tails  of  mixtures 
of  iron,  sodium  and  other  metallic  vapors.  At  times 
the  same  comet  will  have  two  or  more  tails  of  dif- 
ferent types. 

Since  the  material  driven  off  from  the  nucleus  or 
head  of  a  comet  by  electrical  repulsion  and  light-pres- 
sure is  never  recovered,  it  is  evident  that  comets  are 
continually  disintegrating.  Also,  comets  that  have 
passed  close  to  the  sun  at  perihelion  have  frequently 
been  so  disrupted  by  tidal  forces  that  one  nucleus  has 
separated  into  several  parts  and  the  newly  formed 
nuclei  have  pursued  paths  parallel  to  the  original  orbit, 
each  nucleus  developing  a  tail  of  its  own. 

Many  periodic  comets,  it  is  now  known,  have  gradu- 
ally been  broken  up  and  dissipated  into  periodic  swarms 
of  meteors  as  a  result  of  the  disruptive  effect  produced 
by  too  frequent  returns  to  the  vicinity  of  the  sun. 

These  swarms  of  meteors  continue  to  travel  around 
the  sun  in  the  orbits  of  the  former  comets.  The  earth 
encounters  a  number  of  such  swarms  every  year  at 
certain  definite  times. 

The  largest  and  best  known  of  these  swarms  or 
showers  are  the  Leonids,  which  appear  about  November 
15;  the  Andromedas  (or  Bielids),  which  appear  later  in 
the  same  month  and  the  Perseids,  which  appear  early 


COMETS  169 

in  August.  These  swarms  are  named  for  the  constella- 
tions in  which  their  "radiant"  lies,  that  is,  the  point  in 
the  heavens  from  which  they  appear  to  radiate.  The 
position  of  the  radiant  depends  upon  the  direction 
from  which  the  swarm  is  coming.  It  is  simply  a 
matter  of  perspective  that  the  individual  particles  ap- 
pear to  radiate  from  the  one  point,  for  they  are  actually 
travelling  in  parallel  lines. 

The  luminosity  of  these  meteoric  particles  is  caused 
by  the  friction  produced  by  their  passage  through  the 
atmosphere.  They  always  appear  noiselessly  because 
they  are  mere  particles  of  meteoric  dust  weighing  at  the 
most  scarcely  a  grain.  They  differ  greatly  in  this 
respect  from  their  large  and  noisy  relatives,  the  meteor- 
ites, bolides  and  fireballs. 

Numberless  small  meteoric  particles  are  entrapped 
by  the  earth's  atmosphere  every  day.  They  are  re- 
ferred to  as  "shooting"  stars  or  "falling"  stars  though, 
of  course,  they  are  not  in  any  sense  stars.  It  is  only 
when  these  meteoric  particles  travel  in  well-defined 
cornet ary  orbits  and  appear  at  certain  definite  times 
every  year  that  they  are  referred  to  as  swarms  or 
showers  of  meteors. 

The  luminosity  of  comets  is  due  not  only  to  reflected 
sunlight,  but  to  certain  unknown  causes  that  produce 
sudden  and  erratic  increases  or  decreases  of  brilliancy. 
The  causes  of  these  sudden  changes  in  luminosity  are 
unknown;  possibly  electrical  discharges  or  chance  col- 
lisions between  fragments  of  considerable  size  may  ac- 
count for  some  of  them. 


170      ASTRONOMY  FOR  YOUNG  FOLKS 

The  peculiar  behavior  of  the  tails  of  comets  at  cer- 
tain times  has  frequently  been  noted  and  suggests  the 
existence  of  quantities  of  finely-divided  meteoric  or 
gaseous  matter  within  the  solar  system  that  has  no 
appreciable  effect  upon  the  huge  planetary  masses,  but 
offers  sensible  resistance  to  the  passage  of  the  tenuous 
gases  of  which  the  tails  of  comets  are  composed.  The 
fact  that  the  earth  daily  encounters  meteoric  dust, 
meteorites  and  fireballs  also  indicates  that  meteoric 
matter  exists  in  considerable  quantities  within  our 
solar  system.  Tails  of  comets  appear  at  times  to  be 
twisted  or  brushed  aside  as  if  they  had  encountered 
some  unknown  force  or  some  resisting  medium. 

Up  to  the  present  time  several  hundred  comets  have 
been  discovered.  Nearly  three-fourths  of  this  number 
travel  in  orbits  that  appear  to  be  parabolas.  Of  the 
remaining  number  there  are  about  forty  that  have 
been  "captured"  by  the  major  planets,  Jupiter,  Saturn, 
Uranus  and  Neptune,  though  Jupiter  possesses  the 
lion's  share  of  these  captured  comets.  Scarcely  a  year 
passes  by  that  several  comets  are  not  discovered.  Most 
of  these  are  telescopic,  however,  even  when  they  are 
near  the  sun  and  at  their  greatest  brilliancy.  Naked- 
eye  comets  of  great  splendor  and  brilliancy  are  com- 
paratively rare  and  there  has  been  a  particular  dearth 
of  such  unusual  comets  during  the  past  thirty  years 
or  iso. 

The  last  spectacular  comet,  unless  we  make  an  ex- 
ception of  Halley's  periodic  comet,  which  made  its  re- 
turn according  to  prediction  in  1910,  was  the  great 


COMETS  171 

comet  of  1882  which  was  visible  in  broad  daylight 
close  to  the  sun  and  at  its  perihelion  passage  swept 
through  the  solar  corona  with  a  velocity  that  exceeded 
two  hundred  and  fifty  miles  a  second  and  carried  it 
through  one  hundred  and  eighty  degrees  of  its  orbit 
in  less  than  three  hours. 

Some  comets  approach  much  closer  to  the  sun  than 
others.  The  majority  of  all  comets  observed  have 
come  within  the  earth's  orbit  and  no  known  comet 
has  its  perihelion  beyond  the  orbit  of  Jupiter.  It 
is,  of  course,  possible  that  there  may  be  a  number  of 
comets  that  never  come  within  the  orbit  of  Jupiter, 
but  it  is  very  unlikely  that  any  such  comet  would  ever 
be  discovered.  The  majority  of  comets  are  simply 
small,  fuzzy  points  of  light  that  are  only  visible  tele- 
scopically  and  the  greater  the  perihelion  distance  of  the 
comet  the  less  likely  is  it  to  be  seen  with  the  naked  eye. 

Since  comets  as  well  as  planets  obey  Kepler's  first 
law,  known  as  the  law  of  areas,  and  sweep  over  equal 
areas  in  equal  times,  it  is  evident  that  when  a  comet  is 
at  perihelion,  or  nearest  to  the  sun,  it  is  moving  at  maxi- 
mum speed  and  when  it  is  at  aphelion,  or  farthest  from 
the  sun,  it  is  moving  at  minimum  speed.  Moreover, 
its  speed  at  these  two  points  in  its  orbit  varies  tre- 
mendously since  the  orbits  of  comets  are  ellipses  of 
very  high  eccentricity.  The  speed  with  which  the  plan- 
ets are  traveling  is,  on  the  other  hand,  remarkably 
uniform  since  their  orbits  are  nearly  circular. 

The  leisurely  speed  with  which  a  comet  travels 
through  the  frigid  outer  regions  of  the  solar  system 


172      ASTRONOMY  FOR  YOUNG  FOLKS 

is  gradually  accelerated  as  the  comet  draws  nearer  and 
nearer  to  the  sun  until  it  has  acquired  near  the  time  of 
perihelion  passage  a  velocity  that  occasionally  exceeds 
two  hundred  miles  a  second.  Here,  also,  the  great  in- 
crease in  light  and  heat  and  the  strong  magnetic  field  of 
the  sun  produce  complex  changes  in  the  gaseous  and 
meteoric  substances  of  which  the  comet  is  composed 
until  the  characteristic  tail  and  peculiar  cometary  fea- 
tures are  fully  developed.  As  the  comet  again  recedes 
from  the  sun  after  perihelion  passage  its  speed  slackens 
once  more.  It  soon  parts  with  its  tail  and  other  spec- 
tacular features  and  fades  rapidly  from  view  even  in 
the  largest  telescopes. 


XXIII 

METEORITES 

METEORITES,  bolides  or  fireballs,  as  they  are  vari- 
ously called,  are  stones  that  fall  to  the  earth  from 
the  heavens.  They  furnish  the  one  tangible  evidence 
that  we  possess,  aside  from  that  furnished  by  the  spec- 
troscope, as  to  the  composition  of  other  bodies  in  space 
and  it  is  a  significant  fact  that  no  unknown  elements 
have  ever  been  found  in  meteorites,  though  the  forms 
in  which  they  appear  are  so  characteristic  that  they 
make  these  stones  readily  distinguishable  from  stones 
of  terrestrial  origin. 

The  origin  of  meteorites  is  not  definitely  known,  but 
the  evidence  is  very  strong  in  favor  of  the  theory  that 
they  are  the  larger  fragments  of  disintegrated  comets 
of  which  meteors  and  shooting  stars  are  the  smaller; 
the  distinction  between  the  two  being  simply  that  the 
latter  class  includes  all  bodies  that  are  completely  con- 
sumed by  friction  with  the  earth's  atmosphere  and, 
therefore,  only  reach  the  surface  in  the  form  of  mete- 
oric dust. 

According  to  other  theories  meteorites  may  be  frag- 
ments of  shattered  worlds  that  have  chanced  to  come 
too  near  to  a  larger  body  and  have  been  disrupted,  or 

173 


174      ASTRONOMY  FOR  YOUNG  FOLKS 

they  may  possibly  be  the  larger  fragments  of  the  dis- 
integrated comets  of  which  the  meteoric  swarms  are 
the  smaller. 

Interplanetary  space  is  not  altogether  a  void.  Our 
own  planet  sweeps  up  in  the  course  of  a  single  day,  it 
has  been  estimated,  approximately  twenty  million  shoot- 
ing stars  or  meteors  of  sufficient  size  to  be  visible  to 
the  naked  eye,  while  the  estimate  for  the  telescopic  par- 
ticles runs  up  to  four  hundred  million. 

Meteorites  on  the  other  hand  are  comparatively  rare. 
On  the  average  it  has  been  estimated  about  one  hun- 
dred meteorites  strike  the  earth  in  the  course  of  a  year, 
of  which  number  only  two  or  three  are  actually  seen. 
According  to  Bulletin  94,  U.  S.  National  Museum,  ap- 
proximately six  hundred  and  fifty  falls  and  finds  of 
meteorites  have  been  reported,  representatives  of  which 
appear  in  museums  and  private  collections. 

Meteorites,  as  well  as  shooting  stars  and  meteors, 
frequently  appear  in  showers.  In  such  instances  the 
fall  usually  consists  of  several  hundred  or  thousand  in- 
dividual stones  and  the  area  over  which  they  fall  is 
several  square  miles  in  extent  and  roughly  ellipsoidal 
in  shape.  One  of  the  most  remarkable  of  such  falls 
occurred  at  L'Aigle,  France,  in  1803.  Between  two 
hundred  thousand  stones,  varying  in  weight  from  fifteen 
pounds  to  a  small  fraction  of  an  ounce,  fell  near  Pul- 
tusk,  Poland.  Another  remarkable  fall  of  meteorites 
occurred  at  L'Aigle,  France,  in  1803.  Bet  wen  two 
thousand  and  three  thousand  stones  fell  over  an  ellip- 
soidal area  of  six  and  two-tenths  miles  in  greatest  diam- 


METEORITES  175 

eter,  the  aggregate  weight  of  the  stones  being  not  less 
than  seventy-five  pounds. 

This  fall  of  stones  is  of  particular  interest  since  it 
took  place  at  a  time  when  men  were  still  very  doubtful 
as  to  whether  or  not  stones  actually  fell  to  earth  from 
the  heavens. 

After  this  fall  had  occurred  in  a  most  populous  dis- 
trict of  France  in  broad  daylight  and  attended  by  violent 
explosions  that  lasted  for  five  or  six  minutes  and  were 
heard  for  a  distance  of  seventy-five  miles,  no  reason- 
able doubt  could  longer  be  held  as  to  the  actuality  of 
such  phenomena. 

Meteorites  are  without  exception  of  an  igneous  na- 
ture, that  is,  they  are  rocks  that  have  solidified  from  a 
molten  condition.  They  can  be  classified  into  three 
groups,  Aerolites  or  Stony  Meteorites,  Siderolites  or 
Stony-iron  Meteorites,  and  Siderites  or  Iron  Meteorites. 

More  iron  meteorites  seem  to  have  fallen  in  Mexico 
and  Greenland  than  in  any  other  part  of  the  world — 
at  least  of  its  land  surface. 

Yet  strange  to  say,  of  all  the  meteorites  that  have  been 
seen  to  fall  only  nine  belong  to  the  group  of  Siderites 
or  Iron  Meteorites,  though  the  three  largest  meteorites 
known,  Peary's  meteorite  from  Cape  York,  Greeland, 
weighing  37^  tons,  the  meteorite  lying  on  the  plain 
near  Bacubirito,  Mexico,  weighing  about  20  tons,  and 
the  Willametter,  Oregon,  meteorite,  weighing  15^2  tons 
all  belong  to  this  group.  Moreover,  all  the  Canyon 
Diablo  meteorites,  which  are  strewn  concentrically 
around  Coon  Mountain  crater  in  northern  Arizona  to  a 


176      ASTRONOMY  FOR  YOUNG  FOLKS 

distance  of  about  five  miles,  are  members  of  this  same 
group.  Coon  Mountain  or  Meteor  crater  itself  is  a 
perfectly  round  hole,  about  six  hundred  feet  deep  and 
over  four  thousand  feet  in  diameter  and  was  formed, 
it  is  believed,  by  the  impact  of  a  huge  meteorite  which 
has  never  been  found.  It  is  believed  that  the  Canyon 
Diablo  meteorites,  of  which  there  are  nearly  four  hun- 
dred individuals  in  the  U.  S.  National  Museum  alone, 
were  all  members  of  this  same  fall.  It  is  possible  that 
these  meteorites  of  the  Canyon  Diablo  district,  with 
the  huge  meteorite  that  produced  the  crater  itself,  formed 
the  nucleus  of  a  comet  that  struck  the  earth  not  more 
than  five  thousand  years  ago,  according  to  the  geologi- 
cal evidence. 

All  iron  meteorites  or  siderites  (from  the  Greek 
sideros,  iron)  are  composed  chiefly  of  alloys  of  nickel 
and  iron.  The  percentage  of  nickel  in  these  iron  mete- 
orites is  very  small,  usually  from  five  to  ten  per  cent., 
while  the  iron  forms  about  ninety  or  ninety-five  per 
cent,  of  the  whole.  Cobalt  is  also  present  in  practically 
all  iron  meteorites  in  small  quantities  of  1  per  cent, 
or  less.  Usually  small  quantities  of  iron  sulphide  and 
phosphide  as  well  as  graphite  or  some  other  form  of 
carbon  appear  in  the  iron  meteorites  and  in  some  in- 
stances black  and  white  diamonds  have  been  found,  as 
in  some  of  the  Canyon  Diablo  irons. 

A  very  interesting  and  beautiful  feature  of  many 
iron  meteorites  is  the  Widmanstatten  figures  which  ap- 
pear when  a  section  of  such  a  stone  is  polished  and 
treated  by  means  of  a  weak  acid.  These  figures  are  due 


METEORITES  177 

to  the  unequal  solubility  of  the  three  different  alloys  of 
nickel  and  iron  of  which  the  stones  are  composed. 
The  irons  giving  the  Widmanstatten  figures  are  known 
as  octahedral  irons.  Other  irons  known  as  hexahedral 
irons  give  figures  of  a  different  type  known  as  Neumann 
figures  when  the  polished  section  is  treated  with  weak 
acid,  while  other  irons  are  so  homogeneous  in  their 
composition  that  they  show  no  figures  at  all. 

Aerolites  or  Stony  Meteorites  occur  more  abundantly 
than  iron  or  stony-iron  types,  and  they  are  classified 
into  many  divisions  and  subdivisions  according  to  their 
composition.  In  these  stones  appear  certain  compounds 
that  are  commonly  met  with  in  terrestrial  igneous 
rocks.  The  mineral  that  is  most  abundant  in  the  stony 
meteorites,  composing  sometimes  nearly  seventy-five 
per  cent,  of  the  stone,  is  a  magnesium  and  iron  silicate 
known  as  olivine,  which  is  also  usually  present  in  ter- 
restrial rocks  of  an  igneous  nature.  Certain  compounds 
found  in  the  stony  meteorites  are  rarely  if  ever  found 
in  terrestrial  rocks,  however,  and  these  serve  to  dis- 
tinguish the  stony  meteorites  readily  from  stones  of 
terrestrial  origin.  The  alloys  of  iron  and  nickel,  for 
instance,  that  occur  in  minor  quantities  in  the  stony 
meteorites  and  make  up  usually  about  ninety-five  per 
cent,  of  the  mass  of  the  iron  meteorites,  are  never  found 
in  terrestrial  rocks.  Although  about  thirty  of  the  ter- 
restrial elements  are  to  be  found  in  meteorites,  the 
forms  and  compounds  in  which  they  appear  are  so  char- 
acteristic and  on  the  whole  so  different  from  those 
occurring  in  terrestrial  rocks,  that  the  analyst  has  no 


178      ASTRONOMY  FOR  YOUNG  FOLKS 

difficulty  in  distinguishing  between  the  two.  There  are, 
for  instance  certain  formations  known  as  chondrules, 
peculiar  spherical  and  oval  shapes,  varying  in  size  from 
minute  particles  to  objects  the  size  of  walnuts,  appear- 
ing in  many  varieties  of  stony  meteorites  that  are  never 
found  in  terrestrial  rocks,  and  that  are  one  of  the  most 
puzzling  features  associated  with  the  origin  and  nature 
of  these  stones.  Sometimes  the  chondrules  are  so 
loosely  embedded  in  the  stone  that  they  fall  away  when 
it  is  broken.  In  some  instances  almost  the  entire  stone 
is  made  up  of  these  chondrules.  According  to  one 
theory  the  chondrules  were  originally  molten  drops, 
like  fiery  rain,  and  their  internal  structure,  which 
is  greatly  varied,  depends  upon  their  conditions  of 
cooling. 

Stony  meteorites,  in  which  these  chondrules  are  to 
be  found,  are  spoken  of  as  chondrites.  There  are 
white  and  gray  and  black  chondrites  and  crystalline  and 
carbonaceous  chondrites,  according  to  the  nature  of  the 
chondrules  found  in  the  stones. 

Stony  meteorites  also  contain  minute  quantities  of 
iron  and  nickel  alloys  in  the  form  of  drops  or  stringers. 

Upon  entering  the  earth's  atmosphere  stony  meteor- 
ites become  coated  with  a  thin  black  crust  which  is 
a  glass  formed  by  the  fusion  of  its  surface  materials 
by  the  heat  generated  during  its  passage  through  the 
atmosphere. 

In  many  of  the  stony  meteorites  there  also  appear 
fine  thread-like  veins  which  are  due  to  the  fracturing 
of  the  stone  prior  to  its  entrance  into  the  atmosphere. 


METEORITES  179 

The  material  filling  these  veins  is  coal  black  in  color, 
opaque  and  of  an  unknown  composition. 

Many  meteorites  show  signs  of  collisions  and  en- 
counters with  other  meteorites  outside  of  the  atmos- 
phere as  would  be  expected  as  they  travel  in  swarms 
and  groups.  Sometimes  the  entire  meteorite  is  com- 
posed of  fragments  of  two  or  more  distinct  stones 
cemented  together.  Such  a  stone  is  spoken  of  as  a 
breccia. 

In  the  third  class  of  meteorites  to  which  we  now 
come,  known  as  the  stony-iron  meteorites,  there  is  a 
network  or  sponge  of  nickel-iron  alloy,  the  interstices 
of  which  are  filled  with  stony  material. 

When  this  network  or  sponge  is  continuous  the 
meteorite  is  spoken  of  as  a  stony-iron  pallasite.  When 
the  network  of  metal  is  more  or  less  disconnected  the 
meteorite  is  a  meso-siderite. 

If  meteorites  are  heated  in  a  vacuum,  the  conditions 
existing  in  interplanetary  space  being  thus  produced 
to  a  certain  extent,  they  give  forth  their  occluded  gases 
and  it  has  been  found  that  these  gases  give  spectra 
identical  with  the  spectra  of  certain  comets.  Meteoric 
irons  give  forth  hydrogen  as  their  characteristic  gas 
while  the  gases  occluded  in  the  stony  meteorites  are 
chiefly  the  oxides  of  carbon,  carbon  monoxide  and  car- 
bon dioxide.  It  has  been  found  that  the  amount  of 
gases  contained  in  a  large  meteorite  or  shower  of 
meteorites  is  sufficient  to  form  the  tail  of  a  comet. 
These  facts  all  tend  to  strengthen  the  belief  that  me- 
teorites are  indeed  cometary  fragments. 


180      ASTRONOMY  FOR  YOUNG  FOLKS 

In  view  of  the  fact  that  some  geologists  believe  me- 
teorites may  be  fragments  of  other  worlds,  it  is  of 
interest  to  know  that  so  far  no  fossil-bearing  meteorites 
have  been  found,  and  if  meteorites  are  fragments  of  a 
shattered  world,  such  worlds  must  have  been  reduced 
to  a  molten  condition  at  the  time  of  the  catastrophe. 

The  rapid  passage  of  the  meteorite  through  the  air 
leaves  a  partial  vacuum  in  its  trail  into  which  rush  the 
molecules  of  air  from  all  sides,  producing  the  character- 
istic noises  that  accompany  the  passage  of  a  meteorite, 
which  have  been  variously  compared  to  the  rattle  of 
artillery,  the  distant  booming  of  cannons  or  the  rumble 
of  thunder. 

There  may  be,  also,  explosions  of  inflammable  gases 
occluded  in  the  crevices  of  the  meteorite  which  will 
shatter  it  into  fragments  or  the  meteorite  may  be  shat- 
tered by  the  resistance  and  pressure  of  the  atmosphere 
or  as  a  result  of  the  extremes  of  temperature  existing 
between  the  interior  and  its  surface.  Many  meteorites 
have  actually  been  seen  to  burst  into  fragments  in  the 
air  with  a  loud  report. 

There  is  practically  no  foundation  for  the  belief  that 
germs  of  life  have  been  brought  to  our  planet  on  such 
igneous  rocks.  No  microscopic  examinations  of  me- 
teorites have  yielded  any  results  that  could  be  inter- 
preted in  favor  of  such  a  view. 

Falls  of  meteorites  are  accompanied  in  nearly  every 
instance  by  terrific  explosions  and  sharp  reports  that 
can  be  heard  for  many  miles  around,  often  causing  the 
ground  to  shake  as  in  an  earthquake.  The  meteorite 


METEORITES  181 

itself  has  been  described  as  resembling  a  ball  of  fire 
or  the  headlight  of  a  locomotive,  and  is  followed  fre- 
quently by  a  trail  of  light  or  a  cloud  of  smoke.  At 
the  time  it  enters  our  atmosphere  a  meteorite  is  moving 
with  planetary  velocity  ranging  from  two  to  forty-five 
miles  per  second.  Its  interior  is  intensely  cold,  ap- 
proaching in  temperature  the  absolute  zero  of  inter- 
planetary space,  and  it  is,  therefore,  far  more  brittle 
than  it  would  be  at  ordinary  temperatures.  As  it 
ploughs  its  way  into  the  earth's  atmosphere  its  surface 
temperature  is  soon  raised  by  friction  to  at  least  3,000° 
or  4,000°  C,  which  is  sufficient  to  fuse  all  surface 
materials  into  the  characteristic  black  crust,  with  which 
stony  meteorites  are  coated. 

Meteorites  are  usually  first  seen  at  an  altitude  of  fifty 
or  sixty  miles.  Although  they  are  moving  with  a  velocity 
comparable  to  that  of  the  planets,  when  they  enter  the 
earth's  atmosphere,  this  velocity  is  so  rapidly  reduced 
by  friction  with  the  atmosphere  that  they  usually  drop 
to  the  surface  of  the  earth  with  a  velocity  about  equal 
to  that  of  ordinary  falling  objects. 

The  flight  of  a  meteorite  often  extends  over  a  path 
several  hundred  miles  in  length  and  the  meteorite  may 
be  seen  by  many  observers  in  several  different  States 
and  yet  finally  fall  in  some  unknown  spot  and  never  be 
found. 

The  evidence  gathered  regarding  the  actual  fall  of 
meteorites  is  often  contradictory.  Some  stones  are 
too  hot  to  handle  for  hours  after  they  fall,  others  are 
merely  warm,  while  still  others  have  been  picked  up  cool 


182      ASTRONOMY  FOR  YOUNG  FOLKS 

or  even  intensely  cold.  Meteorites  have  been  seen  to 
fall  upon  dried  grass  and  upon  straw  without  produc- 
ing even  charring  effects.  The  evidence  regarding  the 
depths  to  which  meteorites  penetrate  the  ground  is 
quite  as  conflicting.  The  largest  of  all  the  stony  me- 
teorites which  fell  at  Krnyahinya,  Hungary,  weighed 
647  pounds  and  buried  itself  to  a  depth  of  eleven  feet. 
Yet  Peary's  Cape  York  iron  meteorite,  weighing  37jA 
tons,  was  only  partially  covered  and  showed  no  signs 
of  abrasions  of  surface  resulting  from  the  fall 

The  Williamette  iron  meteorite,  weighing  16^  tons, 
lay  in  a  forest  when  found  and  was  not  deeply  buried. 
The  Bacubirito  iron  meteorite,  weighing  20  tons,  lay  in 
soft  soil,  barely  beneath  the  level  of  the  surface.  On 
the  other  hand  a  fragment  of  a  stony-iron  meteorite, 
weighing  437  pounds,  that  fell  at  Estherville,  Iowa, 
buried  itself  eight  feet  in  stiff  clay. 

Geologists  in  charge  of  the  meteoric  collections  of 
various  museums  quite  frequently  have  stones  sent  to 
them  for  analysis  that  are  reputed  to  be  of  celestial 
origin.  More  often  than  not  such  stones  are  found 
to  be  purely  terrestrial  in  their  origin.  The  composi- 
tion of  a  meteorite  is  so  characteristic  and  unique  that 
such  a  stone  can  never  be  mistaken.  Finds  of  bona- 
fide  meteorites  are  on  the  whole  extremely  rare. 

It  is  also  a  peculiar  fact  that  meteorites  are  usually 
observed  in  the  months  when  ordinary  meteors  or 
periodic  swarms  of  meteors  are  least  prevalent,  that  is 
in  the  months  of  May,  June  and  July. 


XXIV 
THE  EARTH  AS  A  MAGNET 

TF  a  small,  freely  suspended  compass  needle  is  moved 
•*•  over  a  highly  magnetized  steel  sphere,  it  will  be  seen 
that  it  constantly  changes  its  position  both  horizontally 
and  vertically  so  as  to  lie  always  along  the  "lines  of 
force"  of  the  sphere. 

There  will  be  one  point  on  the  sphere  which  we  will 
call  the  North  Magnetic  Pole,  where  the  north-seeking 
end  of  the  needle  will  point  vertically  downward  or 
make  a  "dip"  of  90°  with  the  tangent  plane.  At  the 
diametrically  opposite  point  on  the  sphere,  called  the 
South  Magnetic  Pole,  the  opposite  end  of  the  compass, 
the  south-seeking  end,  will  point  vertically  downward; 
while  at  a  point  midway  between  the  magnetic  poles 
of  the  sphere  the  needle  will  lie  parallel  to  the  diameter 
connecting  the  two  poles  and  there  will  be  no  dip. 

The  total  intensity  of  the  magnetic  field  surrounding 
the  sphere  will  be  found  to  be  greatest  in  the  vicinity 
of  the  magnetic  poles  and  least,  midway  between  the 
poles. 

Now,  a  freely  suspended  compass  needle  carried  to 
all  parts  of  the  earth  will  behave  very  much  in  the 
same  manner  as  the  needle  moved  over  the  magnetized 
steel  sphere.  There  are  two  points  on  the  earth's  surface, 

183 


184      ASTRONOMY  FOR  YOUNG  FOLKS 

known  as  the  North  and  South  Magnetic  Poles,  where 
the  needle  points  vertically  downward  and  approximately 
midway  between  is  the  Magnetic  Equator  where  the 
compass  needle  places  itself  in  a  perfectly  horizontal 
position  and  the  "dip"  of  the  needle  is  zero.  In  other 
words,  the  earth  acts  as  a  huge  magnet  and  possesses 
a  magnetic  field  with  lines  of  force  converging  towards 
its  poles  similar  to  the  lines  of  force  of  the  steel  sphere. 

There  are,  however,  some  very  important  differences 
between  the  sphere  of  steel  and  our  earth.  The  matter 
of  which  the  earth  is  composed  is  not  homogeneous. 
It  is  believed  to  possess  an  iron  core  of  considerable 
size,  it  is  true,  but  its  outer  shell  is  composed  of  hete- 
rogeneous masses  that  in  certain  regions  cause  very  ap- 
preciable local  deflections  of  the  needle.  It  is  sur- 
rounded, moreover,  by  an  atmosphere  permeated  by 
electrified  particles  of  matter  shot  forth  from  the  sun, 
which  we  now  know  is  a  still  greater  magnet  surrounded 
by  a  magnetic  field  that  is  of  the  order  of  50  gausses  at 
the  poles  and  about  eighty  times  more  powerful  than 
that  of  the  earth. 

It  is  now  a  well-established  fact  that  the  sun's  mag- 
netic field  exerts  a  powerful  influence  over  the  condi- 
tion of  the  earth's  magnetic  field,  and  that  vast  solar 
disturbances  affect  very  materially  the  direction  and  in- 
tensity of  the  lines  of  force. 

It  is  thus  little  wonder  that  this  non-homogeneous  and 
rapidly  rotating  terrestrial  globe,  surrounded  by  an 
electrified  atmosphere  and  subject  to  the  action  of  a  still 
more  powerful  magnet,  the  sun,  should  not  behave  in  a 


THE  EARTH  AS  A  MAGNET  185 

manner  exactly  analagous  to  a  uniformly  magnetized 
steel  sphere. 

The  earth's  magnetic  poles  are  neither  symmetrically 
placed  nor  absolutely  fixed  in  position.  There  is  every 
reason  to  suspect  that  they  shift  about  from  year  to 
year,  and  possibly  fluctuate  irregularly  in  position  in 
the  course  of  a  few  days  or  hours  under  the  influence 
of  disturbing  forces.  The  position  of  the  earth's  North 
Magnetic  Pole,  last  visited  by  Amundsen  in  1903,  now 
lies  approximately  in  Latitude  70°  N.  Longitude  97° 
W.  The  position  erf  the  South  Magnetic  Pole,  according 
to  the  latest  determinations,  is,  in  round  numbers,  in 
Latitude  73°  S.  and  Longitude  156  °  E.  of  Greenwich. 
It  is  evident,  therefore,  that  the  magnetic  poles  of  the 
earth  are  not  symmetrically  placed  and  that  they  lie 
fully  30°  from  the  geographical  poles.  The  chord  con- 
necting the  magnetic  poles  passes  750  miles  from  the 
earth's  center,  and  it  is  about  1,200  miles  from  the  geo- 
graphic pole  to  the  nearest  magnetic  pole.  There 
exist,  moreover,  in  high  latitudes  local  magnetic  poles, 
due  possibly  to  heavy  local  deposits  of  ore.  One  such 
pole  was  discovered  at  Cape  Treadwell,  near  Juneau, 
Alaska,  during  Dr.  L.  A.  Bauer's  observations  there  in 
1900  and  1907.  In  the  center  of  the  observing  tent 
at  this  point  the  needle  pointed  vertically  downward  and 
the  compass  reversed  its  direction  when  carried  from 
one  side  of  the  tent  to  the  other. 

It  is  a  well-known  fact  that  there  are  very  few  points 
on  the  earth's  surface  where  the  compass  needle  points 
either  to  the  true  geographical  pole  or  to  the  magnetic 


186   ASTRONOMY  FOR  YOUNG  FOLKS 

pole,  and  if  it  does  chance  to  do  so,  it  is  a  transient 
happening.  The  "variation  of  the  compass"  or  the  dec- 
lination of  the  needle,  as  it  is  called,  is  the  angle  that 
the  compass  needle  makes  with  the  true  north  and  south 
line  or  the  meridian.  It  is  an  angle  of  greatest  impor- 
tance to  navigators  and  explorers,  for  it  gives  them 
their  bearings,  yet  it  is  unfortunately  subject  to  ceaseless 
variations  of  a  most  complicated  nature,  since  it  de- 
pends on  the  constantly  pulsating  and  never  ceasing 
magnetic  changes  that  sweep  over  the  surface  of  the 
earth  and  through  its  crust.  It  is  affected  by  long 
period  or  secular  changes,  as  they  are  called,  progress- 
ing more  or  less  regularly  in  obscure  cycles  of  unknown 
period.  It  is  subject  to  a  diurnal  change  that  depends 
on  the  position  of  the  sun  relative  to  the  meridian,  and 
that  varies  with  the  seasons  and  with  the  hour  of  the 
day.  It  is  affected  by  the  sun  spot  cycle  of  11.3  years 
which  has  a  direct  effect  upon  the  intensity  of  the  earth's 
magnetic  field.  The  intensity  of  the  magnetic  field  in 
sun  spots  is,  according  to  Abbot,  sometimes  as  high  as 
4,500  gausses  or  9,000  times  the  intensity  of  the  earth's 
field.  At  times  of  maximum  spottedness  of  the  sun  the 
intensity  of  the  earth's  magnetic  field  is  reduced. 

Moreover,  when  great  and  rapidly  changing  spots 
appear  upon  the  sun,  electrified  particles  are  shot  forth 
from  the  sun  with  great  velocity  and  in  great  numbers, 
and  are  drawn  in  towards  the  magnetic  poles  of  the 
earth.  Meeting  the  rarefied  gases  of  the  earth's  upper 
atmosphere,  they  illuminate  them  as  electric  discharges 
illuminate  a  vacuum  tube.  Some  of  these  electrons 


THE  EARTH  AS  A  MAGNET  187 

are  absorbed  by  gases  at  high  elevations,  other  de- 
scend to  lower  levels.  The  most  penetrating  rays  have 
been  known  to  descend  to  an  altitude  of  twenty-five 
miles  which  is  about  the  lowest  limit  yet  found  for 
auroral  displays.  It  is  the  passage  of  these  rays  through 
the  atmosphere  that  cause  the  magnetic  disturbances 
known  as  magnetic  storms,  that  are  associated  with 
the  appearance  of  great  sun  spots  and  auroral  displays. 
At  such  times  sudden  changes  take  place  in  the  intensity 
of  the  earth's  magnetic  field  that  cause  the  compass 
needle  to  shiver  and  tremble  and  temporarily  lose  its 
directive  value.  These  magnetic  storms  have  been 
known  to  produce  great  temporal  changes  in  the  in- 
tensity of  the  earth's  field.  According  to  Dr.  L.  A. 
Bauer,  Director  of  the  Department  of  Terrestrial  Mag- 
netism of  the  Carnegie  Institute  of  Washington,  the 
earth's  intensity  of  magnetization  was  altered  by  about 
one-twentieth  or  one-thirtieth  part  by  the  magnetic 
storm  of  September  25,  1909,  which  was  one  of  the 
most  remarkable  on  record,  and  the  earth's  magnetic 
condition  was  below  par  for  fully  three  months  after- 
wards as  a  result. 

In  addition  to  these  various  regular  and  irregular 
changes  in  the  variation  of  the  compass,  or  declination 
of  the  needle,  due  to  changes  in  the  earth's  magnetic 
field  as  a  whole,  there  are  local  effects  due  to  restricted 
regional  disturbances  of  the  earth's  magnetic  field  or 
to  local  deposits  of  ore,  or  to  volcanoes  or  other  local 
causes.  The  effect  of  all  these  disturbances  upon  the 
declination  of  the  needle  must  be  determined  by  con- 


188      ASTRONOMY  FOR  YOUNG  FOLKS 

tinual  magnetic  surveys  of  all  portions  of  the  earth's 
surface. 

As  a  whole  the  earth's  magnetic  field  is  more  uniform 
over  the  oceans  than  over  the  land,  with  all  its  disturb- 
ing topographical  features.  Yet  this  advantage  is  offset 
largely  in  navigation  by  the  fact  that  every  steel  ship 
that  sails  the  seas  is  a  magnet,  with  its  two  magnetic 
poles  and  its  neutral  line  where  the  two  opposite  mag- 
netic forces  are  neutralized,  as  is  the  case  with  every 
magnet  The  direction  in  which  a  steel  ship  lies  with 
reference  to  the  earth's  magnetic  field  while  it  is  being 
built  determines  the  position  of  the  magnetic  poles  in  its 
hull  and  the  position  of  its  neutral  line  and  this  dis- 
tribution of  magnetism  over  a  ship's  hull  must  be  taken 
account  of  in  the  installation  of  its  standard  compass. 
Every  piece  of  horizontal  and  vertical  iron  aboard  ship 
has  an  effect  upon  the  variation  of  the  compass  and 
compensation  must  be  made  for  such  disturbing  forces. 
The  direction  of  sailing,  the  position  in  which  a  ship 
lies  at  dock,  storms  encountered  at  sea,  the  firing  of 
batteries  (on  warships)  are  some  of  the  factors  that  are 
operative  in  producing  changes  in  the  variation  of  the 
magnetic  compass  aboard  a  ship. 

Every  ship  must  undergo  at  frequent  intervals  mag- 
netic surveys  for  the  purpose  of  determining  its  mag- 
netic constants  and  its  "Table  of  Deviations  of  the 
Compass." 

The  direction  in  which  the  compass  needle  points 
aboard  ship  is  the  resultant  of  the  effect  of  the  earth's 
magnetic  field  and  the  magnetic  field  of  the  ship,  and 


THE  EARTH  AS  A  MAGNET  189 

both  fields  are  subject  to  continual  and  complicated 
variations  from  year  to  year,  from  day  to  day,  and 
even  from  hour  to  hour! 

The  elements  of  the  earth's  magnetic  field  are  de- 
termined for  any  one  epoch  by  long-continued  magnetic 
surveys  carried  on  to  a  greater  or  less  extent  by  the 
various  nations  of  the  world,  and  the  results  are  pub- 
lished in  the  form  of  magnetic  charts  for  land  and  sea, 
showing  the  values  of  the  three  magnetic  elements,  dec- 
lination of  the  needle,  dip  or  inclination,  and  horizontal 
intensity  of  the  earth's  field  for  a  definite  period.  So 
rapid  are  even  the  long-period  changes  in  the  earth's 
magnetic  field  that  a  magnetic  chart  can  be  relied  upon 
for  only  a  very  few  years  and  fresh  data  for  the  con- 
struction of  these  charts  that  are  so  valuable  to  naviga- 
tors and  explorers  must  be  gathered  continually. 

The  Department  of  Terrestrial  Magnetism  of  the 
Carnegie  Institute  of  Washington  is  engaged  in  coiv 
tinual  magnetic  surveys  of  the  earth  by  land  and  sea 
that  are  of  the  highest  value  not  only  to  navigators  but 
also  to  scientists  interested  in  solving  the  great  and 
mysterious  problem  of  the  underlying  causes  of  the 
earth's  magnetism. 

To  give  an  idea  of  the  extent  and  scope  of  the  work 
of  this  department  it  may  be  mentioned  that  its  non- 
magnetic ship  Carnegie  made  in  the  period  1909-1918 
a  total  run  of  189,176  nautical  miles,  nearly  nine  times 
the  earth's  circumference,  with  an  average  day's  run  of 
119  nautical  miles.  Magnetic  observations  were  made 
practically  every  day  at  a  distance  of  100  to  150  miles 


190      ASTRONOMY  FOR  YOUNG  FOLKS 

apart.  In  this  nine-year  period  five  cruises  were  made. 
On  her  first  cruise  the  Carnegie  sailed  from  St.  John's 
Newfoundland,  to  Falmouth,  England,  over  practically 
the  same  course  followed  by  the  famous  astronomer, 
Halley,  in  the  Paramour  Pink  two  centuries  earlier  to 
determine  the  variation  of  the  compass.  In  her  fourth 
voyage  the  Carnegie  circumnavigated  the  world  in  sub- 
antarctic  regions  in  118  days — a  record  time.  She 
has  traversed  all  oceans  from  80°  North  to  the  parallel 
of  60°  South  and  has  crossed  and  recrossed  her  own 
path  and  the  path  of  her  predecessor,  the  Galilee,  many 
times,  thus  making  it  possible  to  determine  for  the 
points  of  intersection  the  secular  changes  in  the  mag- 
netic elements. 

After  a  thorough  overhauling  in  1919  and  the  instal- 
lation of  a  four-cylinder  gasoline  engine,  made  of 
bronze  throughout,  to  take  the  place  of  the  producer- 
gas  engine  used  on  earlier  cruises,  the  Carnegie  started 
on  her  sixth  cruise  with  a  crew  of  twenty-three  officers 
and  men  on  October  9,  1919.  A  cruise  of  61,500  miles 
was  planned  in  the  South  Atlantic,  Indian  and  Pacific 
Oceans  to  last  approximately  two  years.  Unsurveyed 
regions  in  the  South  Atlantic  and  Indian  Ocean  were  to 
be  covered  and  the  route  was  planned  so  as  to  ob- 
tain a  large  number  of  observations  of  the  progressive 
changes  that  have  taken  place  in  the  magnetic  elements. 
This  is  accomplished  as  stated  above  by  intersecting 
former  routes  and  obtaining  new  values  of  the  ele- 
ment at  the  points  of  intersection. 

In  addition  to  its  ocean  magnetic  surveys  the  Depart- 


THE  EARTH  AS  A  MAGNET  191 

ment  of  Terrestrial  Magnetism  also  carries  on  exten- 
sive land  surveys  in  all  parts  of  the  globe.  In  1919 
special  expeditions  were  sent  out  by  the  Department  to 
observe  the  total  solar  eclipse  of  May  29th  at  stations 
distributed  over  the  entire  zone  of  visibility  of  the 
eclipse  and  immediately  outside.  At  Dr.  Bauer's  station 
in  Liberia  the  total  phase  was  visible  in  a  cloudless 
sky  for  more  than  six  minutes,  which  is  very  close  to 
the  maximum  length  of  phase  that  can  possibly  be  ob- 
served. Unmistakable  evidence  was  gathered  at  all 
stations  of  an  appreciable  variation  in  the  earth's  mag- 
netic field  during  a  solar  eclipse,  which  variation  is 
the  reverse  of  that  causing  the  daylight  portion  of  the 
solar  diurnal  variation  of  the  needle. 

In  addition  to  the  magnetic  survey  work  on  land  and 
sea  which  is  the  chief  work  of  the  Department  of  Ter- 
restrial Magnetism,  atmospheric-electric  observations  are 
carried  on  continually  on  land  and  sea  and  experiments 
have  been  carried  on  at  Langley  Field,  Va.,  lately,  in 
the  development  of  methods  and  instruments  for  deter- 
mining the  geographical  position  of  airplanes  by  astron- 
omical observations.  There  has  also  been  recently 
formed  under  this  department  a  Section  of  Terrestrial 
Electricity. 

The  cause  of  the  earth's  magnetic  field  is  still  one  of 
the  greatest  unsolved  problems  of  astro-physics.  The 
theory  that  has  been  advanced  by  Schuster  that  all  large 
rotating  masses  are  magnets  as  a  result  of  their  rota- 
tion has  received  considerable  attention  from  astrophy- 
sicists, and  attempts  have  been  made  to  prove  this  ex- 


192      ASTRONOMY  FOR  YOUNG  FOLKS 

perimentally.  It  has  been  found  that  iron  globes  spun 
at  high  velocities  in  the  laboratory  do  not  exhibit  mag- 
netic properties.  This  may  mean  simply  that  the  mag- 
netic field  is  too  weak  to  be  detected  in  the  case  of  a 
comparatively  small  iron  sphere  spun  for  a  limited 
period  under  laboratory  conditions.  It  must  be  remem- 
bered that  the  earth  has  been  rotating  rapidly  on  its  axis 
for  millions  of  years  and  is,  compared  to  terrestrial 
objects,  an  extremely  large  mass.  Yet  it  has  been  shown 
that  as  a  whole  our  earth  is  an  extremely  weak  magnet, 
and  that  if  it  were  made  entirely  of  steel  and  mag- 
netized as  highly  as  an  ordinary  steel-bar  magnet, 
the  magnetic  forces  at  its  surface  would  be  a  thou- 
sand times  greater  than  they  actually  are. 

If  it  is  true  that  all  rotating  bodies  are  magnets, 
then  all  the  heavenly  bodies,  planets,  suns  and  nebulae 
are  surrounded  by  magnetic  fields.  We  know  nothing 
to  the  contrary.  In  fact,  we  know  this  to  be  true  for 
the  earth  and  sun,  and  strongly  suspect  that  it  is  so  in 
the  case  of  the  planets  Jupiter  and  Saturn. 

When  we  understand  more  about  the  properties  of 
matter,  the  nature  of  magnetism,  as  well  as  of  gravity 
may  be  revealed  to  us. 


XXV 

SOME   EFFECTS   OF   THE  EARTH'S   ATMOS- 
PHERE UPON  SUNLIGHT 

IT  is  impossible  to  exaggerate  the  importance  of 
the  atmosphere  to  all  forms  of  life  upon  the  sur- 
face of  the  earth.  If  there  were  no  atmosphere  there 
would  be  no  life,  because  it  is  through  the  agency  of 
the  water-vapor,  carbon-dioxide  and  oxygen  in  the  at- 
mosphere that  all  life-processes  are  maintained. 

If  there  were  no  atmosphere  there  would  not  only 
be  no  life  upon  the  earth;  there  would  be  also  none 
of  the  beautiful  color  effects  produced  by  the  passage 
of  sunlight  through  the  atmosphere.  There  would  be 
no  blue  skies,  no  beautiful  sunrise  and  sunset  effects, 
no  twilight,  no  rainbows,  no  halos,  no  auroral  displays, 
no  clouds,  no  rains,  no  rivers  nor  seas,  no  winds  nor 
storms.  The  heavens  would  be  perfectly  black  except 
in  the  direction  of  the  heavenly  bodies  which  would 
shine  as  brilliantly  by  day  as  by  night. 

To  understand  how  the  atmosphere  produces  color 
effects  such  as  blue  skies,  sunrise  and  sunset  tints,  rain- 
bows and  halos,  as  well  as  the  twinkling  of  the  stars, 
and  numerous  other  phenomena,  we  must  know  some- 
thing of  the  nature  of  light  itself. 

193 


194      ASTRONOMY  FOR  YOUNG  FOLKS 

Light  moves  outward  from  any  source,  such  as  the 
sun,  in  all  directions  radially,  or  along  straight  lines 
(so  long  as  it  does  not  encounter  a  gravitational  field) 
with  the  unimaginable  velocity  of  186,000  miles  per 
second.  As  it  advances  it  vibrates  or  oscillates  back 
and  forth  across  its  path  in  all  directions  at  right 
angles  to  this  path,  unless  it  is  plane  polarized  light, 
in  which  case  its  vibrations  are  confined  to  one 
plane  only. 

These  vibrations  or  oscillations  of  light  take  the  form 
of  a  wavelike  motion,  one  wave-length  being  the 
distance  passed  over  in  the  time  of  one  vibration, 
measured  from  crest  to  crest  or  from  trough  to 
trough  of  adjacent  waves. 

We  may  consider  that  a  beam  or  ray  of  sunlight 
is  made  up  of  a  great  number  of  individual  rays  of 
different  wave-lengths  and  different  colors.  The  aver- 
age wave-length  of  light,  the  wave-length  of  the  green 
ray  in  sunlight,  is  about  one-fifty-thousandth  part  of 
an  inch,  that  is,  it  would  take  about  fifty-thousand 
wave-lengths  of  green  light  to  cover  a  space  of  one 
inch.  Now,  since  light  makes  one  vibration  in  passing 
over  a  distance  of  one  wave-length,  it  makes  fifty 
thousand  vibrations,  while  advancing  one  inch  and 
since  it  advances  one  hundred  and  eighty-six  thousand 
miles  in  one  second  we  can  easily  figure  out  that  a 
ray  of  sunlight  of  average  wave-length  makes  about 
six  hundred  trillion  vibrations  (600,000,000,000,000) 
in  a  single  second! 

The  chief  colors  of  which  sunlight  or  white   light 


EFFECTS  OF  EARTH'S  ATMOSPHERE      195 

is  composed  are  red,  orange,  yellow,  green,  blue, 
indigo  and  violet,  though  there  are  an  infinite  number 
of  gradations  of  color  which  blend  into  one  another, 
gradually  producing  the  intermediate  tints  and  shades. 
The  colors  just  mentioned  are  called  the  primary 
colors  of  the  solar  spectrum,  which  can  be  produced 
as  a  band  of  light  of  variegated  colors,  arranged  in 
the  order  named  by  passing  a  ray  of  ordinary  sunlight 
through  a  glass  prism.  The  individual  jays  of 
different  color  and  wave-length  that  make  up  a  beam 
of  sunlight,  or  white  light,  then  separate  out  in  the 
order  of  the  wave-lengths.  The  red  rays  vibrate  the 
most  slowly  and,  have  the  longest  wave-length  of 
all  the  rays  of  the  visible  spectrum.  About  four 
hundred  trillion  vibrations  of  red  light  reach  the  eye 
in  one  second.  Violet  rays,  on  the  other  hand,  vibrate 
the  most  rapidly  of  all  the  visible  rays  and  have  the 
shortest  wave-length.  About  eight  hundred  trillion 
vibrations  of  violet  light  reach  the  eye  every  second. 
The  wave-lengths  of  the  intermediate  colors  decrease 
in  length  progressively  from  the  red  to  the  violet  and, 
of  course,  the  frequencies  of  their  vibrations  increase 
in  the  same  order.  All  sunlight  is  made  up  of  these 
rays  of  different  colors  and  different  vibration  fre- 
quencies, and  of  other  rays  as  well,  to  which  the  human 
eye  is  not  sensitive,  and  which,  therefore,  do  not 
appear  in  the  visible  spectrum.  Among  these  invisible 
rays  are  the  infra-red  rays  which  come  just  below 
the  red  of  the  visible  spectrum,  and  which  are  of 
longer  wave-length  than  the  red  rays,  and  the  ultra- 


196      ASTRONOMY  FOR  YOUNG  FOLKS 

violet  rays,  which  lie  beyond  the  violet  rays  of  the 
visible  spectrum,  and  are  of  shorter  wave-length  than 
the  violet  rays. 

Now  a  ray  of  ordinary  sunlight  is  separated  into 
the  rays  of  various  colors,  which  form  the  solar  spec- 
trum when  it  passes  from  a  medium  of  one  density 
obliquely  to  a  medium  of  another  density,  as  when  it 
passes  from  air  to  glass,  or  from  air  to  water,  or  from 
outer  space  into  the  earth's  atmosphere.  Under  such 
circumstances  its  velocity  is  slowed  down  when  it 
passes  from  a  rare  to  a  denser  medium,  and  the  waves 
of  different  wave-lengths  are  bent  from  their  former 
course,  or  refracted,  by  different  amounts.  The  red 
rays,  of  longest  wave-length,  are  bent  from  their  former 
course  the  least,  and  the  violet  rays,  of  shortest  wave- 
length, are  bent  the  most  upon  passing  from  a  rare 
to  a  denser  mediutm.  As  a  result  the  ray  of  sunlight 
is  spread  out  or  dispersed  into  its  rays  of  different 
wave-length  and  color  upon  entering  a  medium  of 
different  density.  It  is  this  refraction  and  dispersion 
of  sunlight  that  produces  many  color  effects  in  the 
earth's  atmosphere. 

The  atmosphere  is  not  of  uniform  density  throughout. 
At  high  altitudes  it  is  extremely  rare.  That  is,  there 
is  little  of  it  in  a  given  volume.  Close  to  the  earth's 
surface,  however,  it  is  comparatively  dense.  Half  of 
all  the  atmosphere  is  within  three  and  one-half  miles  of 
the  surface  and  half  of  the  remainder  lies  within  the 
next  three  and  a  half  miles.  We  mav  consider  it  as 


EFFECTS  OF  EARTH'S  ATMOSPHERE     197 

made  up,  on  the  whole,  of  layers  of  different  densities, 
strongly  compressed  near  the  surface. 

Imagine  a  ray  of  sunlight  entering  the  earth's  at- 
mosphere from  without.  If  it  comes  from  a  point  in 
the  zenith  its  course  is  not  changed  upon  entering  the 
atmosphere,  because  light  passing  from  a  certain 
medium — as  space — into  a  medium  of  different  density, 
is  not  bent  from  its  course,  or  refracted,  provided  it 
enters  the  new  medium  in  a  direction  perpendicular  to 
the  surface.  If  it  enters  the  atmosphere  (which  is 
the  new  medium  of  greater  density)  obliquely,  refrac- 
tion, or  bending  of  the  ray,  takes  place,  and  as  the 
ray  advances  toward  the  earth,  through  layers  of 
increasing  densities,  it  is  bent  from  its  former  course 
more  and  more.  As  the  advancing  rays  of  different 
colors  and  wave-lengths  in  the  beam  of  sunlight  are 
slowed  down  in  the  new  medium,  the  red  rays  are 
turned  from  their  course  the  least  an,d  the  violet  rays 
the  most  and  the  entire  advancing  wave-front  of  the 
beam  of  sunlight  is  bent  down  more  and  more  toward 
the  horizon,  as  it  proceeds  through  the  atmosphere. 
As  we  on  the  earth's  surface  see  the  ray  not  along 
its  bent  course  through  the  atmosphere,  but  in  the 
direction  in  which  it  finally  enters  our  eyes,  the  effect 
of  refraction  upon  a  ray  of  light  passing  through  the 
atmosphere  is  to  displace  the  object  in  the  direction 
of  the  zenith  or  increase  its  distance  above  the  horizon. 
As  a  result  of  refraction  we  see  the  sun — or  moon — 
above  the  western  horizon  after  it  has  really  set,  and 
above  the  eastern  horizon  before  it  has  really  risen. 


198      ASTRONOMY  FOR  YOUNG  FOLKS 

The  oval  shape  that  the  sun,  or  moon,  often  presents 
on  rising  or  setting,  is  due  to  the  fact  that  the  light 
from  the  lower  limb  is  passing  through  denser  air 
than  the  light  from  the  upper  limb,  and  so  is  refracted 
more.  As  a  result  the  lower  limb  is  lifted  propor- 
tionately more  than  the  upper  limb.  This  distorts  the 
form  of  the  solar  or  lunar  disk,  making  it  appear  oval 
instead  of  circular. 

The  familiar  twinkling  or  scintillation  of  stars  and, 
more  rarely,  of  the  planets,  is  a  result  of  interference 
of  light  waves  due  to  irregular  and  variable  refraction 
in  air  that  is  not  uniform  in  density,  owing  to  the 
presence  of  constantly  rising  and  descending  atmos- 
pheric currents  of  different  densities.  This  also  pro- 
duces the  shimmering  or  unsteadiness  of  star  images 
in  the  telescope,  that  interferes  so  greatly  with  accu- 
rate measurements  of  angles  or  observations  of  plan- 
etary markings. 

One  may  ask  why  it  is,  if  light  from  an  object, 
say  a  star,  is  bent  from  its  course  and  separated  into 
rays  of  various  colors  upon  entering  the  earth's  at- 
mosphere, that  we  do  not  see  the  object  drawn  out 
into  a  band  of  spectral  colors.  It  is  because  the  angular 
separation  of  the  various  colors  is  so  slight  under 
ordinary  circumstances  that  light  from  one  point  is 
blended  with  light  from  a  neighboring  point  of  com- 
plementary color  to  produce  white  light  again.  Under 
certain  circumstances,  however,  beautiful  color  effects 
may  be  seen  in  the  earth's  atmopshere  as  a  result  of 
the  refraction  of  sunlight. 


EFFECTS  OF  EARTH'S  ATMOSPHERE     199 

The  blue  color  of  the  sky  and  its  brightness  is 
caused  by  the  scattering  of  the  rays  of  shortest  wave- 
length, the  violet  and  blue  rays,  by  the  oxygen  and 
nitrogen  in  the  upper  atmosphere.  The  molecules  of 
these  gases  interfere  with  the  passage  of  these  rays, 
powerfully  scattering  and  dispersing  them,  and  thus 
increasing  the  length  of  their  path  through  the  air 
and  diffusing  their  color  and  brightness  in  the  upper 
atmosphere,  while  permitting  rays  of  longer  wave- 
length, the  red  and  orange,  to  pass  on  practically 
undisturbed. 

When  an  object  in  the  heavens  lies  close  to  the 
horizon,  the  rays  of  light  from  it  have  to  travel  a 
longer  path  through  the  atmosphere  than  when  the 
object  is  overhead,  and  that  too  through  the  densest 
part  of  the  atmosphere,  which  lies  close  to  the  earth's 
surface,  and  in  which  are  floating  many  dust  particles 
and  impurities  from  the  earth's  surface.  All  these 
particles,  as  well  as  the  increased  density  of  the  at- 
mosphere, interfere  with  the  free  passage  of  the 
rays,  especially  of  shorter  wave-lengths.  The  violet 
and  blue  rays  are  sifted  out  and  scattered  in  their 
long  journey  through  the  lower  strata  of  air,  far 
more  than  when  they  come  to  us  from  an  object  high 
in  the  sky.  Even  the  red  and  yellow  rays  are  more 
or  less  scattered  and  bent  aside — diffracted — by  these 
comparatively  large  particles  near  the  surface.  The 
reddish  color  of  the  sun,  moon  and  even  of  the  stars 
and  planets,  when  seen  near  the  horizon,  as  well  as 
the  beautiful  sunset  tints,  in  which  reds  and  pinks 


200      ASTRONOMY  FOR  YOUNG  FOLKS 

and  yellows  predominate,  are  due  to  the  fact  that  the 
rays  of  longer  wave-length  are  more  successful  in 
penetrating  the  dense,  dust-laden  layers  of  the  lower 
atmosphere.  It  is  to  be  free  of  the  dust  and  impurities 
as  well  as  the  unsteadiness  of  the  lower  atmosphere, 
that  observatories  are  built  at  high  altitudes  when- 
ever possible. 

When  there  have  been  unusually  violent  volcanic 
eruptions,  and  great  quantities  of  finely  divided  dust 
have  been  thrown  into  the  upper  atmosphere,  the  effect 
upon  the  blue  and  violet  rays  from  the  sun  is  very 
great.  The  volcanic  dust  particles  are  so  large  that 
instead  of  scattering  these  rays  of  shorter  wave-length, 
as  do  the  oxygen  and  nitrogen  in  our  atmosphere, 
they  reflect  them  back  into  space  and  so  decrease  the 
amount  of  light  and  heat  received  from  the  sun.  For 
this  reason  the  general  temperature  of  the  earth  is 
lowered  by  violent  volcanic  eruptions.  Unusually 
cold  periods,  that  lasted  for  months,  followed  the  terri- 
ble eruption  of  Krakatoa  in  1883  and  of  Katmai  in 
1912. 

At  times  when  much  dust  is  present  in  tne  atmos- 
phere, the  sky  is  a  milky  white  color  by  day  as  a 
result  of  the  reflection  of  sunlight  from  the  dust 
particles.  Sunrise  and  sunset  colors  are  then  particu- 
larly gorgeous,  with  reds  predominating.  At  such 
times  the  blue  and  violet  rays  are  almost  completely 
shut  out,  and  the  red,  orange  and  yellow  rays  are 
powerfully  diffracted  and  scattered  by  the  dust  par- 
ticles in  the  air, 


EFFECTS  OF  EARTH'S  ATMOSPHERE     201 

The  twilight  glow  that  is  visible  for  some  time 
before  sunrise  or  after  sunset  is,  of  course,  entirely 
an  atmospheric  effect  caused  by  the  reflection  of 
sunlight  to  our  eyes  from  the  upper  atmosphere,  upon 
which  the  sun  shines,  while  it  is,  itself,  concealed  from 
our  view  below  the  horizon.  The  atmosphere  ex- 
tends in  quantities  sufficient  to  produce  twilight  to  an 
elevation  of  about  sixty  miles. 

When  all  the  rays  of  which  sunlight  is  composed 
are  reflected  in  equal  proportions  we  get  the  impression 
of  white  light.  Dust  and  haze  in  the  air  reflect  all 
rays  strongly  and  give  a  whitish  color  to  an  otherwise 
blue  sky.  Brilliant  white  clouds  appear  white,  because 
they  are  reflecting  all  rays  equally.  Clouds  or  portions 
of  clouds  appear  black  when  they  are  in  shade  or, 
at  times,  by  contrast  with  portions  that  are  more 
strongly  illuminated,  or  when  they  are  moisture-laden 
and  near  the  point  of  saturation,  when  they  are  ab- 
sorbing more  light  than  they  reflect.  At  sunrise  and 
sunset,  when  the  light  that  falls  upon  the  clouds  is 
richest  in  red  and  orange  and  yellow,  clouds  reflect 
these  colors  to  our  eyes,  and  we  see  the  brilliant 
sunset  hues  which  are  more  intense  the  more  the 
air  is  filled  with  dust  and  impurities. 

The  familiar  and  beautiful  phenomenon  of  the  rain- 
bow is  produced  by  refraction,  reflection  and  inter- 
ference of  sunlight  by  drops  of  falling  water,  such 
as  rain  or  spray.  As  the  ray  of  sunlight  enters 
the  drop  of  water,  which  acts  as  a  tiny  glass  prism,  it 
is  refracted  or  bent  from  its  course  andi  spread  out 


202      ASTRONOMY  FOR  YOUNG  FOLKS 

into  its  spectral  colors.  Reflection  of  these  rays  next 
takes  place  (once  or  twice,  as  the  case  may  be)  from 
the  inside  of  the  drop  and  a  second  refraction  of 
the  reflected  ray  takes  place  as  it  leaves  the  drop.  The 
smaller  the  drops  the  more  brilliant  is  the  rainbow 
and  the  richer  in  color.  The  most  brilliant  rainbows 
are  produced  by  drops  between  0.2  and  0.4  millimeters 
in  diameter.  In  addition  to  the  primary  bow,  which 
has  a  red  outer  border  with  a  radius  of  42°,  there 
is  the  secondary  bow  with  a  radius  of  about  51°  and 
with  colors  reversed,  the  red  being  on  the  inner  border; 
the  supernumerary  bows  which  are  narrow  bands  of 
red,  or  green  and  red,  appear  parallel  to  the  primary 
and  secondary  bows  along  the  inner  side  of  the  primary 
bow  and  the  outer  side  of  the  secondary  bow.  No 
rainbow  arches  ever  appear  between  the  primary  and 
secondary  bows,  and  it  can  be  shown  in  fact,  that 
the  illumination  between  these  two  bows  is  at  a 
minimum. 

The  primary,  secondary  and  supernumerary  bows 
all  lie  opposite  the  sun  in  the  direction  of  the  ob- 
server's shadow  and  the  observer  must  stand  with 
his  back  to  the  sun  in  order  to  see  them.  The  primary 
and  secondary  rainbow  arches  take  the  form  of  arcs 
of  circles  that  have  their  common  center  on  the 
line  connecting  the  sun  with  the  observer  at  a  point 
as  far  below  the  horizon  in  angular  distance  as  the 
sun  is  above  the  horizon.  It  is,  therefore,  never 
possible  to  see  a  rainbow  arch  of  more  than  a  semi- 
circle in  extent  unless  the  observer  is  at  an  elevation 


EFFECTS  OF  EARTH'S  ATMOSPHERE     203 

above  the  surrounding  country,  under  which  circum- 
stances it  might  be  possible  to  see  a  complete  circle 
formed  by  the  rainbow. 

The  highest  and  longest  arch  appears  when  the  sun 
is  on  the  horizon,  and  the  greater  the  altitude  of  the 
sun  the  smaller  and  lower  the  visible  arch.  As  the 
angular  radius  of  the  primary  bow  is  42°  and  of  the 
secondary  bow  51°  and  as  the  common  center  of  the 
two  circles  is  always  as  far  below  the  horizon  as  the 
sun  is  above,  it  is  never  possible  to  see  either  primary 
of  secondary  rainbow  when  the  altitude  of  the  sun 
is  over  51°,  or  the  primary  bow  when  the  altitude 
is  over  42°.  For  this  reason  rainbows  are  rarely 
seen  at  or  near  noon  in  mid-latitudes,  since  the  sun  is 
usually  at  an  elevation  of  more  than  42°  at  noon, 
especially  in  the  summer  season,  which  is  also  the 
most  favorable  season  for  rainbows,  owing  to  the 
great  likelihood  of  rain  and  sunshine  occurring  at  the 
same  time. 

The  light  which  comes  to  an  observer  from  the 
primary  bow  is  once  reflected  within  the  drop,  and  that 
which  comes  from  the  secondary  bow  is  twice  re- 
flected within  the  drop.  The  sharper  and  brighter  light 
therefore  comes  from  the  primary  bow  of  42°  radius. 
The  space  between  the  two  bows  is  particularly  dark, 
because  it  can  be  shown  that  the  drops  there  do  not 
reflect  any  light  at  all. 

The  rainbow  colors  are  rarely  pure  or  arranged  in 
spectral  order,  owing  to  interference  of  light  waves. 
It  is  the  interference  of  light  waves  from  different 


204      ASTRONOMY  FOR  YOUNG  FOLKS 

parts  of  the  same  drop  that  produces  the  bands  of 
alternate  maximum  and  minimum  brightness,  that  lie 
below  the  primary  bow  and  beyond  the  secondary 
bow.  The  red  or  green  and  red  bands  of  maximum 
brightness  produced  thus  by  interference,  are  called 
the  supernumerary  bows,  and  they  are  always  found 
parallel  to  the  primary  and  secondary  bow  within  the 
former  and  above  the  latter. 

The  distance  of  the  rainbow  from  the  observer  is 
the  distance  of  the  drops  that  form  it.  A  rainbow 
may  be  formed  by  clouds  several  miles  distant  or  by 
the  aid  of  the  garden  hose  on  our  lawn.  No  two 
observers  can  see  exactly  the  same  rainbow  because 
the  rainbow  arch  encircles  the  surface  of  a  cone 
whose  vertex  is  at  the  observer's  eye  and  no  two 
such  vertices  can  exactly  coincide.  Two  observers 
see  rainbows  formed  by  different  drops. 

Refraction  of  light  by  ice-crystals  in  clouds  pro- 
duces many  beautiful  color  effects,  such  as  halos  of 
various  types  around  sun  or  moon,  vertical  light  pil- 
lars, circumzenithal  arcs,  and  parhelia — "sun-dogs" — 
or  paraselense — "moon-dogs" — which  are  luminous 
spots  at  equal  altitudes  with  sun  or  moon — one  to  the 
left  the  other  to  the  right,  at  an  angular  distance 
of  22°. 

The  most  usual  form  of  halo  is  that  of  22°  radius. 
This  is  a  luminous  ring  of  light  surrounding  sun  or 
moon,  with  the  inner  edge  red  and  sharply  defined 
and  the  spectral  colors  proceeding  outward  in  order; 
red  is  frequently  the  only  color  visible,  the  remainder 


EFFECTS  OF  EARTH'S  ATMOSPHERE     205 

of  the  ring  appearing  whitish.  Since  the  halo  is  pro- 
duced by  refraction  of  light  by  ice-crystals,  which 
exist  in  clouds  of  a  certain  type  gathering  at  high 
altitudes,  it  is  always  a  very  good  indicator  of  an 
approaching  storm. 

Coronas  are  luminous  rings  showing  the  spectral 
colors  in  the  reverse  order,  that  is,  with  the  inner  edge 
blue  instead  of  red.  They  are  usually  of  very  small 
radius,  scarcely  two  degrees,  closely  surrounding  sun 
or  moon  and  are  produced — not  by  refraction — but 
by  diffraction  or  a  bending  aside  of  the  rays  as  they 
pass  between — without  entering — very  small  drops  of 
water  in  clouds.  As  in  the  case  of  refraction,  the  red 
rays  are  turned  from  their  course  the  least  and  the 
violet  rays  the  most. 

Many  of  these  phenomena — halos,  luminous  spots, 
vertical  pillars  and  arcs  of  light  may,  at  times,  be 
seen  simultaneously,  when  clouds  of  ice-crystals  are 
forming  around  the  sun  or  moon.  They  then  pre- 
sent a  very  complex  and  beautiful  outline  of  luminous 
circles,  arches  and  pillars  that  have  a  mysterious  and 
almost  startling  appearance  when  the  cause  is  not 
clearly  understood. 

We  have  found  then  that  sunlight  is  made  up  of 
rays  of  many  different  wave-lengths  and  colors  and 
that  the  atmosphere  acts  upon  these  rays  in  various 
ways.  It  reflects  them  or  turns  them  back  on  their 
course;  it  refracts  them  as  they  pass  through  the  gases 
of  which  the  atmosphere  consists,  or  through  the 
water- vapor  and  ice-crystals  suspended  in  it,  thus  sifting 


206      ASTRONOMY  FOR  YOUNG  FOLKS 

out  and  dispersing  the  rays  of  different  colors  and 
wave-lengths  and  producing  beautiful  color  effects;  it 
diffracts  them  or  bends  them  aside  as  they  pass  between 
the  fine  dust  particles  and  small  drops  of  water  in 
the  air;  again  sifting  out  the  rays  of  different  colors 
and  producing  color  effects  similar  to  those  pro- 
duced by  refraction;  it  also  scatters  and  disperses, 
through  the  action  of  the  molecules  of  oxygen  and 
nitrogen  in  the  upper  strata,  the  blue  and  violet  rays 
of  shorter  wave-length  and  thus  produces  the  blue 
color  and  brightness  of  the  sky;  it  produces  beautifully 
colored  auroral  streamers  and  curtains  and  ray?  of 
light  through  the  electrical  discharges  resulting  when 
the  rarefied  gases  in  the  upper  air  are  bombarded  by 
electrified  particles  shot  forth  from  the  sun. 

It  is  our  atmosphere,  then,  that  we  have  to  thank 
for  all  these  beautiful  displays  of  color  that  delight 
our  eyes  and  give  pleasure  to  our  existence,  as  well 
as  for  the  very  fact  of  our  existence  upon  a  planet  that 
without  its  presence  would  be  an  uninhabitable  waste, 
covered  only  with  barren  rocks. 


XXVI 

KEEPING  TRACK  OF  THE  MOON 

OF  all  celestial  objects  the  nearest  and  most  familiar 
is  our  satellite,  the  moon.     Yet  the  mistakes  and 
blunders   that  otherwise   intelligent   persons    frequently 
make  when  they   refer  to  the  various   aspects  of  the 
moon  are  quite  unbelievable. 

Who  has  not  read  in  classics  or  in  popular  fiction 
of  crescent  moons  riding  high  in  midnight  skies,  of  full 
moons  rising  above  western  cliffs  or  setting  beyond 
eastern  lakes?  Who  has  not  seen  the  moon  drawn 
in  impossible  positions  with  horns  pointing  toward  the 
horizon,  or  a  twinkling  star  shining  through  an  ap- 
parently transparent  moon? 

Careful  observation  of  the  moon  in  all  its  various 
phases  and  at  different  seasons  is  the  best  method 
to  be  used  in  acquiring  a  knowledge  of  the  elementary 
facts  regarding  the  motion  of  the  moon  through  the 
heavens  from  day  to  day,  but  that  requires  that  one 
be  up  often  after  midnight  and  in  the  early  hours 
preceding  dawn  and  so  it  is  that  we  feel  so  hazy  in 
regard  to  what  happens  to  the  moon  after  it  has 
passed  the  full. 

A  few  fundamental  rules  can  be  easily  acquired, 
however,  and  these  will  enable  us  to  locate  the  moon 

207 


208      ASTRONOMY  FOR  YOUNG  FOLKS 

in  the  right  quarter  of  the  heavens  at  any  time  of  the 
day  or  night  when  we  know  its  phase  and  the  approxi- 
mate position  of  the  sun  at  the  same  instant,  and  thus 
we  may  avoid  some  of  the  most  obvious  blunders  that 
are  made  in  dealing  with  the  general  aspect  of  the 
moon  at  any  given  time. 

As  can  be  verified  by  direct  observation,  the  moon 
is  always  moving  continually  eastward.  Since  it  makes 
a  complete  revolution  around  the  earth  from  new 
moon  back  to  new  moon  again  in  a  little  less  than 
thirty  days,  it  passes  over  about  twelve  degrees  a 
day  (360°  divided  by  30),  on  the  average,  or  one-half 
a  degree  an  hour,  which  is  about  the  angular  extent 
of  its  own  diameter.  Therefore  every  hour  the  moon 
moves  eastward  a  distance  equal  to  its  own  diameter. 
This  is  of  course  only  approximate  as  the  moon  moves 
more  rapidly  in  some  parts  of  its  orbit  than  in  others. 

In  addition  to  its  real  eastward  motion  the  moon 
shares  the  apparent  daily  westward  motion  of  all 
celestial  objects  which  is  due  to  the  daily  rotation  of 
the  earth  on  its  axis  in  the  opposite  direction.  That 
is,  the  moon,  as  well  as  the  sun,  stars  and  planets, 
rises  in  the  east  and  sets  in  the  west  daily.  On 
account  of  its  continuous  eastward  motion,  however, 
the  moon  rises  later  every  night,  on  the  average 
about  fifty  minutes,  though  the  amount  of  this  daily 
retardation  of  moon-rise  varies  from  less  than  half 
an  hour  to  considerably  over  an  hour  at  different 
seasons  of  the  year  and  in  different  latitudes.  In  the 
course  of  a  month  then  the  moon  has  risen  at  all  hours 


KEEPING  TRACK  OF  THE  MOON       209 

of  the  day  and  night  and  set  at  all  hours  of  the  day 
and  night. 

It  might  seem  unnecessary  to  emphasize  the  fact 
that  the  moon  always  rises  in  the  east  were  it  not 
that  the  astronomer  occasionally  meets  the  man  who 
insists  that  he  has  at  times  seen  the  moon  rise  in 
the  west. 

To  be  sure  the  new  crescent  moon  first  becomes 
visible  above  the  western  horizon  shortly  after 
sunset  though  it  rises  in  the  east  the  morning  of  the 
same  day  shortly  after  sunrise.  As  is  also  true  of 
the  sun  the  exact  point  on  the  horizon  where  the 
moon  rises  or  sets  varies  from  day  to  day  and  from 
season  to  season.  In  one  month  the  moon  passes  over 
very  nearly  the  same  path  through  the  heavens  that 
the  sun  does  in  one  year,  for  the  moon's  path  is  in- 
clined only  five  degrees  to  the  ecliptic  or  apparent  path 
of  the  sun  through  the  heavens.  It  can  never  pass 
more  than  28^°  (23j4°  +  5°)  south  of  the  celestial 
equator,  nor  more  than  28J^°  north  of  it.  It  has 
a  slightly  greater  range  in  altitude  than  the  sun, 
therefore.  North  of  28^°  north  latitude  it  always 
crosses  the  meridian  south  of  the  zenith  and  below 
28J^°  south  latitude  it  crosses  the  meridian  north 
of  the  zenith.  In  tropical  regions  the  moon  sometimes 
passes  north  of  the  zenith,  sometimes  south,  or  again 
directiy  through  the  zenith. 

Since  the  full  moon  is  always  diametrically  opposite 
to  the  sun  it  passes  over  nearly  the  same  part  of  the 
heavens  that  the  sun  did  six  months  before.  In 


210      ASTRONOMY  FOR  YOUNG  FOLKS 

winter  then  when  the  sun  is  south  of  the  equator 
the  moon  "rides  high"  at  night  north  of  the  equator 
and,  vice  versa,  in  summer  when  the  sun  is  north  of 
the  equator  the  full  moon  "rides  low"  south  of  the 
equator.  In  winter  then  we  have  more  hours  of  moon- 
light than  we  have  in  summer.  This  may  be  of  no 
great  advantage  in  mid-latitudes  but  we  may  imagine 
what  a  boon  it  is  to  the  inhabitants  of  the  Arctic 
and  Antarctic  regions  to  have  the  friendly  moon 
above  the  horizon  during  the  long  winter  months  when 
the  sun  is  never  seen  for  days  at  a  time. 

At  time  of  "new"  moon  the  moon  lies  directly 
between  us  and  the  sun,  but  ordinarily  passes  just 
to  the  north  or  south  of  the  sun  since  its  orbit  is 
inclined  five  degrees  to  the  ecliptic  or  plane  of  the 
earth's  orbit.  If  the  moon's  path  lay  exactly  in  the 
ecliptic  we  would  have  an  eclipse  of  the  sun  every 
month  at  new  moon  and  an  eclipse  of  the  moon  two 
weeks  later  at  full  moon.  Now  the  moon  crosses 
the  ecliptic  twice  a  month,  the  points  of  crossing 
being  called  the  nodes  of  its  orbit,  but  only  twice  a 
year  is  the  moon  nearly  enough  in  line  with  the  sun 
at  the  time  it  crosses  to  cause  eclipses.  Every  year, 
then,  there  are  two  "eclipse  seasons,"  separated  by 
intervals  of  six  months,  when  the  moon  is  in  line  with 
the  sun  at  or  close  to  the  point  where  it  crosses  the 
ecliptic;  then  and  only  then  can  solar  and  lunar  eclipses 
occur.  The  solar  eclipses,  of  course,  will  occur  when 
the  moon  is  new,  that  is,  when  the  moon  passes  directly 
between  ithe  earth  and  the  sun  and  throws  its  shadow 


KEEPING  TRACK  OF  THE  MOON       211 

over  the  earth ;  and  the  lunar  eclipses  two  weeks 
later  when  the  earth  passes  between  the  sun  and  moon 
and  throws  its  shadow  over  the  face  of  the  moon. 

Probably  there  is  no  astronomical  subject  that 
has  been  more  generally  misunderstood  than  that  of 
solar  and  lunar  eclipses.  It  is  well  to  remember  that 
solar  eclipses  can  only  occur  at  time  of  new  moon  and 
lunar  eclipses  only  at  the  time  of  full  moon;  and  at 
the  time  of  eclipses,  whether  lunar  or  solar,  the  moon 
is  at  or  near  its  nodes,  the  points  where  its  orbit 
crosses  the  ecliptic.  There  are  always  at  least  two 
solar  eclipses  every  year  and  there  may  be  as  many 
as  five.  There  are  years  when  there  are  no  lunar 
eclipses,  though  ordinarily  both  solar  and  lunar  eclipses 
occur  every  year,  some  partial  others  total. 

The  moon  shines  only  by  reflected  sunlight.  It  is 
of  itself  a  solid,  dark  body  with  its  day  surface  in- 
tensely hot  and  its  night  surface  intensely  cold,  a 
world  of  extreme  temperatures. 

At  new  moon  all  of  the  night  side  of  the  moon  is 
turned  toward  us,  at  full  moon  all  of  the  day  side.  At 
other  phases  we  see  part  of  the  day  side  and  part  of 
the  night  side  and  the  illuminated  side  of  the  moon 
is  always  the  side  that  is  towards  the  sun.  Failure 
to  observe  this  simple  rule  leads  to  many  grievous 
blunders  in  depicting  the  moon. 

At  the  time  of  new  moon  the  moon,  moving  con- 
tinually eastward,  passes  north  or  south  of  the  sun 
from  west  to  east  except  when  it  passes  directly  in 
front  of  the  sun,  causing  eclipses.  A  day  or  so  later 


212      ASTRONOMY  FOR  YOUNG  FOLKS 

the  waxing  crescent  moon  or  the  "new  moon,"  as  it 
is  popularly  called,  becomes  visible  low  in  the  west 
immediately  after  sunset.  The  moon  is  now  east  of 
the  sun  and  will  remain  east  of  the  sun  until  the  time 
of  full  moon.  During  the  period  from  new  moon  to 
full  moon  it  will,  therefore,  rise  after  the  sun  and 
set  after  the  sun.  The  waxing  crescent  moon  will 
not  be  visible  in  the  morning  hours  because,  inasmuch 
as  it  rises  after  the  sun,  it  is  lost  to  view  in  the  sun's 
brilliant  rays.  Nevertheless,  it  follows  the  sun  across 
the  sky  and  becomes  visible  in  the  west  as  soon  as 
the  sun  has  disappeared  below  the  western  horizon. 
The  thin  illuminated  crescent  has  its  horns  or  cusps 
turned  away  from  the  point  where  the  sun  has  set. 
The  horns  of  the  crescent  can  never  point  toward 
the  horizon  since  the  illuminated  side  of  the  moon  is 
always  turned  toward  the  sun  whether  the  sun  is  above 
or  below  our  horizon. 

As  hour  by  hour  and  day  by  day  the  moon  draws 
farther  eastward  and  increases  its  angular  distance 
from  the  sun,  more  and  more  of  the  illuminated  side 
becomes  visible;  the  crescent  increases  in  width  and 
area  and  the  moon  appears  higher  in  the  western  sky 
each  night  at  sunset. 

Usually  about  seven  and  a  fraction  days  after  the 
date  of  new  moon  the  moon  completes  the  first  quar- 
ter of  its  revolution  around  the  earth.  The  period 
from  one  phase  to  the  next  is  variable  and  irregular, 
being  sometimes  less  than  seven  days  and  at  other 
times  more  than  eight  days,  since  the  moon  does  not 


KEEPING  TRACK  OF  THE  MOON       213 

move  at  a  uniform  rate  in  different  parts  of  its  orbit. 

When  the  moon  has  completed  the  first  quarter 
of  a  revolution  it  is  ninety  degrees  east  of  the  sun 
and  presents  the  phase  known  as  ' 'half -moon"  since 
half  of  the  surface  that  is  turned  toward  the  earth 
is  illuminated  and  half  is  in  darkness.  It  is  said  to 
be  "at  the  first  quarter."  The  illuminated  half  is  of 
course  the  western  half  because  the  sun  is  to  the 
west  of  the  moon.  The  half  moon  is  near  the 
meridian  at  sunset  and  sets  near  midnight.  Up  to 
the  first  quarter,  then,  the  moon  is  a  crescent  in  the 
western  sky  during  the  first  part  of  the  night  and 
should  never  be  represented  as  east  of  the  meridian  or 
near  the  meridian  at  midnight. 

After  the  moon  has  passed  the  first  quarter  and 
before  it  is  full  more  than  half  of  the  side  turned 
toward  the  earth  is  illuminated  and  it  is  in  the  "gib- 
bous" phase.  It  is  still  the  western  limb  that  is  fully 
illuminated.  The  moon  is  now  east  of  the  meridian 
at  sunset  and  it  crosses  the  meridian  before  midnight 
and  sets  before  sunrise.  All  who  are  abroad  during 
the  first  half  of  the  night  find  this  phase  of  the  moon 
more  favorable  to  them  than  the  gibbous  phase  follow- 
ing full  moon. 

The  moon  now  being  above  the  horizon  at  sunset 
is  visible  continuously  from  sunset  to  midnight  but 
sets  some  time  during  the  second  half  of  the  night, 
while  the  full  moon  shines  throughout  the  night, 
rising  in  the  east  at  sunset  and  setting  in  the  west 
at  sunrise. 


214      ASTRONOMY  FOR  YOUNG  FOLKS 

When  the  moon  is  full  it  is  180°  east,  or  west,  of 
the  sun  and  so  both  its  eastern  and  western  limbs 
are  perfectly  illuminated.  After  the  full  the  moon  goes 
through  its  phases  in  reverse  order,  being  first  gibbous, 
then  a  half -moon  once  more,  and  lastly  a  waning 
crescent. 

It  is  now  west  instead  of  east  of  the  sun  and  so 
it  is  the  eastern  limb  that  is  fully  illuminated  by  the 
sun.  Being  west  of  the  sun  it  will  now  rise  before  the 
sun  and  set  before  the  sun,  the  interval  decreasing  each 
day  as  the  moon  draws  in  toward  the  sun  once  more. 

The  gibbous  phase  preceding  full  moon  is  favorable 
to  all  abroad  before  midnight  but  the  gibbous  phase 
following  full  moon  is  more  favorable  to  those  who 
are  abroad  after  midnight,  for  from  full  moon  to  last 
quarter  the  moon  is  below  the  horizon  at  sunset,  and 
of  course,  is  rising  later  and  later  each  night,  while 
at  sunrise  it  is  still  above  the  horizon,  appearing  each 
day  higher  and  higher  above  the  western  horizon  at 
sunrise  as  it  approaches  the  third  or  last  quarter. 

When  it  has  reached  this  point  it  is  once  more  a 
half -moon,  though  now  it  is  the  eastern  half  instead 
of  the  western  half  of  the  disk  that  is  fully  illuminated. 
The  moon  is  90°  west  of  the  sun  at  third  quarter  and 
from  this  phase  to  the  phase  of  new  moon  it  is  a 
crescent  once  -more,  but  now  a  waning  instead  of  a 
waxing  crescent.  It  appears  east  of  the  meridian 
before  sunrise  and  as  the  crescent  grows  thinner  it 
draws  nearer  and  nearer  to  the  eastern  horizon  and 
the  rising  sun.  As  with  the  waxing  crescent  moon 


KEEPING  TRACK  OF  THE  MOON       215 

the  horns  are  turned  away  from  the  horizon.  The 
waning  crescent  moon  is  always  to  be  looked  for  east 
of  the  meridian  and  to  be  associated  with  the  rising 
sun,  while  the  waxing  crescent  .moon  is  to  be  looked 
for  west  of  the  meridian  and  associated  with  the 
setting  sun.  Neither  the  waxing  nor  the  waning 
crescent  moon  will  be  visible  during  the  midnight 
hours. 

As  the  waning  crescent  moon  grows  thinner  and 
draws  in  closer  to  the  sun  each  successive  night,  its 
time  of  rising  precedes  that  of  the  sun  by  an  ever- 
decreasing  interval  until  finally  the  crescent  disappears 
from  view  in  the  eastern  sky;  the  next  day  we  see 
no  crescent  either  in  the  eastern  or  western  skies — 
the  moon  is  once  more  in  conjunction  with  the  sun 
and  "new."  One  revolution  of  the  moon  about  the 
earth  with  respect  to  the  sun  has  been  completed  and 
a  day  or  so  later  we  may  look  for  a  new  crescent 
moon  in  the  western  sky  after  sunset. 


XXVII 
THE  MOTIONS  OF  THE  HEAVENLY  BODIES 

ABOUT  three  hundred  and  twenty  years  ago 
Giordano  Bruno  was  burned  at  the  stake  for  his 
audacity  in  believing  in  the  existence  of  other  worlds. 
A  few  decades  later  the  famous  astronomer  Galileo 
was  forced  to  publicly  recant  his  belief  that  the  earth 
moved.  Yet  the  truth  could  not  long  be  suppressed 
by  such  means,  and  since  those  dark  days  man's  ad- 
vance in  knowledge  has  been  so  rapid  that  it  seems 
to  us  today  in  this  wonderful  age  of  scientific  dis- 
covery almost  inconceivable  that  man  ever  believed  that 
the  earth,  a  tiny  planet  of  a  vast  solar  system,  was 
"the  hub  of  the  universe,"  the  fixed  and  immovable 
center  about  which  revolved  all  the  heavenly  bodies. 
Very  reluctantly,  however,  and  with  bitter  feeling,  but 
in  the  light  of  overwhelming  evidence  man  finally  gave 
up  his  long-cherished  idea  of  terrestrial  importance, 
and  when  finally  forced  to  move  his  fixed  center  of 
the  universe,  he  moved  it  only  so  far  as  the  compara- 
tively nearby  sun. 

This  center  he  then  regarded  as  fixed  in  space  and 
also  held  to  his  belief  that  the  stars,  set  in  an  imaginary 
celestial  sphere,  were  immovable  in  space  as  well,  and 
all  at  the  same  distance  from  the  sun.  So,  scarcely 

216 


SPIRAL  NEBULA  IN  CANES  VENATICI 
Taken  with  6o-inch  Reflector  of  the  Mt.  Wilson  Observatory 


MOTIONS  OF  HEAVENLY  BODIES      217 

two  hundred  years  ago  we  find  that  the  astronomer 
Bradley  was  endeavoring  to  measure  this  common 
distance  of  the  "fixed  stars."  Though  he  failed  in 
this  attempt  he  made  the  important  discovery  that  the 
observed  positions  of  the  stars  are  not  their  true  posi- 
tions, owing  to  the  fact  that  the  velocity  of  light  is 
not  infinite  but  takes  a  definite  finite  interval  of  time 
to  travel  a  given  distance.  As  a  result  the  stars  always 
appear  displaced  in  the  direction  of  the  earth's  motion 
around  the  sun,  the  amount  of  the  displacement  de- 
pending upon  the  velocity  of  the  earth  in  its  orbit 
and  the  velocity  of  light.  This  "aberration  of  light," 
as  it  is  called,  furnished  additional  proof  that  the  earth 
revolves  about  the  sun  and  was  one  more  nail  driven 
into  the  coffin  of  the  old  Ptolemaic  theory  that  the 
earth  was  the  center  of  the  universe.  Bradley  also 
discovered  that  the  positions  of  the  stars  were  affected 
by  the  wabbling  of  the  earth's  axis,  called  its  "nutation." 
Although  in  the  days  of  Bradley  neither  the  methods 
of  observation  nor  the  instruments  were  sufficiently 
accurate  to  show  the  minute  shifts  in  the  positions  of 
the  stars  that  reveal  the  individual  motions  of  the  stars 
and  the  distances  of  those  nearest  to  us,  yet  the  dis- 
covery of  the  two  large  displacements  in  the  positions 
of  all  the  stars,  due  to  the  aberration  of  light  and  the 
nodding  of  the  earth's  axis  were  of  the  greatest  value, 
for  they  were  a  necessary  step  in  the  direction  of  the 
precise  measurements  of  modern  times.  It  is  only 
through  measurements  of  the  greatest  refinement  and 
accuracy  that  it  is  possible  to  detect  the  motions  and 


218   ASTRONOMY  FOR  YOUNG  FOLKS 

distances  of  the  stars  and  to  discover  the  wonderful 
truths  about  the  nature  and  structure  of  the  universe 
that  they  are  revealing  to  us  today. 

After  unsuccessful  attempts  extending  over  several 
centuries  the  distance  of  one  of  the  nearest  stars,  the 
faint  61  Cygni,  as  it  is  catalogued,  was  finally  de- 
termined by  the  astronomer  Bessel  in  the  year  1838. 

This  star  is  about  ten  light-years  distant  from  the 
earth,  which  places  it  about  six  hundred  and  thirty 
thousand  times  farther  away  from  us  than  the  sun; 
that  is,  we  would  have  to  travel  six  hundred  and  thirty 
thousand  times  the  distance  from  the  earth  to  the 
sun  to  reach  this  very  close  stellar  neighbor,  61  Cygni. 
The  nearest  of  all  the  stars,  Alpha  Centauri,  is  over 
two  hundred  and  seventy  thousand  times  the  distance 
from  the  earth  to  the  sun.  It  is,  therefore,  little 
wonder  that  the  early  astronomers  believed  that  the 
stars  were  fixed  in  space  since  even  the  nearest  is 
so  far  away  that,  viewed  from  opposite  points  in  the 
earth's  orbit,  its  apparent  change  in  position  due  to 
our  actual  change  in  position  of  186,000,000  miles, 
amounts  to  only  one  and  a  half  seconds  of  arc.  Two 
stars  separated  by  one  hundred  and  sixty  times  this 
angular  distance  might  possibly  be  glimpsed  as  two 
distinct  stars  by  a  person  with  good  eyesight,  though  to 
most  of  us  they  would  appear  as  one  star.  Upon  the 
measurement  of  such  minute  angles  depended  a  knowl- 
edge of  the  distances  of  the  nearest  stars. 

It  is  to  Sir  William  Herschel  that  we  owe  the  dis- 
covery, more  than  a  hundred  years  ago,  of  the  motion 


MOTIONS  OF  HEAVENLY  BODIES      219 

of  the  sun  through  the  universe.  From  the  consid- 
eration of  a  long  series  of  observations  of  the  posi- 
tions of  the  stars  this  famous  astronomer  discovered 
that  the  stars  in  the  direction  of  the  constellation 
Hercules  were  separated  by  much  greater  angular 
distances  than  the  stars  diametrically  opposite  in  the 
heavens.  In  other  words,  the  stars  were  spreading 
apart  in  one  portion  of  the  heavens  and  crowding 
together  in  the  opposite  direction  and  he  rightly  in- 
terpreted this  to  mean  that  the  sun  was  moving  in  the 
direction  of  the  constellation  of  Hercules.  It  was  not 
until  the  spectroscope  was  applied  to  the  study  of 
the  heavens  in  the  latter  part  of  the  nineteenth  century 
that  the  amount  of  this  motion  of  the  sun  was  found 
to  be  about  twelve  and  a  half  miles  per  second,  or  four 
times  the  distance  from  the  earth  to  the  sun  in 
a  year. 

It  is  to  Sir  William  Herschel  that  we  owe  also  the 
discovery  of  binary  systems  of  stars  in  which  two 
stars  swing  around  a  point  between  them  called  their 
center  of  gravity. 

Our  first  conception  of  the  immensity  and  grandeur 
of  -the  universe  dates  from  the  time  of  the  older 
Herschel  only  a  century  or  so  ago.  The  mysterious 
nebulae  and  star  clusters  were  then  discovered,  the  won- 
ders of  the  Milky  Way  were  explored,  and  a  new  planet 
and  satellites  in  our  own  solar  system  were  discovered. 
It  was  found  that  the  sun  and  the  stars  as  well  as 
the  planets  were  in  motion.  Neither  sun  nor  earth 


220   ASTRONOMY  FOR  YOUNG  FOLKS 

could  be  regarded  any  longer  as  a  fixed  point  in 
the  universe. 

With  the  application  of  the  spectroscope  to  the  study 
of  the  heavens  toward  the  end  of  the  nineteenth  century 
the  key  to  a  treasure-house  of  knowledge  was  placed 
in  the  hands  of  the  astronomers  of  modern  times  and 
as  a  result  we  are  now  learning  more,  in  a  few  decades, 
about  the  wonders  and  mysteries  of  the  heavens  than 
was  granted  to  man  to  learn  in  centuries  of  earlier 
endeavor.  Yet  it  is  the  feeling  of  the  astronomer  of 
today  that  he  is  only  standing  on  the  threshold  of 
knowledge  and  that  the  greatest  of  all  discoveries,  that 
of  the  nature  of  matter  and  of  time  and  space  is  yet 
to  be  made. 

It  is  the  spectroscope  that  tells  us  so  many  wonderful 
facts  about  the  motions  of  the  stars,  nebulae  and  star 
clusters.  It  tells  us  also  practically  all  we  know  about 
the  physical  condition  of  our  own  sun  and  of  the 
other  suns  of  the  universe,  their  temperature  and  age, 
and  the  peculiarities  of  their  atmospheres. 

Some  of  the  most  important  astronomical  discoveries 
that  have  been  made  in  the  past  few  years  have  to 
do  with  the  distribution  and  velocities  of  the  heavenly 
bodies  as  revealed  by  the  spectroscope. 

It  has  been  found,  with  the  aid  of  the  spectroscope, 
that  the  most  slowly  moving  of  all  stars  are  the  ex- 
tremely hot  bluish  Orion  stars  with  an  average  velocity 
of  eight  miles  per  second,  while  the  most  rapidly 
moving  stars  are  the  deep-red  stars  with  an  average 
velocity  of  twenty-one  miles  per  second,  and  there 


MOTIONS  OF  HEAVENLY  BODIES      221 

is  in  all  cases  a  relationship  existing  between  the  color, 
or  spectrum,  of  a  star  and  its  velocity.  The  reason 
for  this  connection  between  the  two  still  remains 
undiscovered. 

The  spectroscope  has  also  told  us  some  astonishing 
facts  in  recent  years  about  the  velocities  of  the 
spiral  nebulae. 

It  is  now  known  that  these  mysterious  objects  are 
moving  with  the  tremendous  average  velocity  of 
four  hundred  and  eighty  miles  per  second,  which  ex- 
ceeds the  average  velocity  of  the  stars  fully  twenty- 
five  fold.  They  possess,  moreover,  internal  motions 
of  rotation  that  are  almost  as  high  as  their  velocities 
through  space.  It  is  now  generally  believed  that  spiral 
nebulae  are  far  distant  objects  of  enormous  size  and 
mass,  exterior  to  our  own  system  of  stars  and  similar 
to  it  in  form. 

In  place  of  the  universe  of  the  "fixed  stars"  and 
the  immovable  sun  or  earth  of  a  few  centuries  ago  we 
find  that  modern  astronomical  discovery  is  substituting 
a  universe  of  inconceivable  grandeur  and  immensity  in 
a  state  of  ceaseless  flux  and  change. 

Our  earth — an  atom  spinning  about  on  its  axis 
and  revolving  rapidly  around  a  huge  sun  that  is  equal 
in  volume  to  more  than  a  million  earths — is  carried 
onward  with  this  sun  through  a  vast  universe  of  suns. 

Only  an  average-sized  star  among  several  hundred 
million  other  stars  is  this  huge  sun  of  ours,  moving 
with  its  planet  family  through  the  regions  of  the 
Milky  Way,  where  are  to  be  found  not  only  moving 


222      ASTRONOMY  FOR  YOUNG  FOLKS 

clusters  and  groups  of  stars,  speeding  along  their 
way  in  obedience  to  the  laws  of  motion  of  the  system 
to  which  they  belong,  but  also  strangely  formed  nebulae 
covering  vast  stretches  of  space,  whirling  and  seething 
internally  and  shining  with  mysterious  light,  and  still 
other  stretches  of  dark  obscuring  matter  shutting  off 
the  rays  of  suns  beyond. 

The  extent  and  form  of  this  enormous  system  of 
stars  and  nebulae  and  the  laws  that  govern  the  motions 
of  its  individual  members  are  among  the  problems 
that  the  astronomers  of  today  are  attempting  to 
solve.  On  both  sides  of  these  regions  of  the  Milky 
Way,  wherein  lies  our  own  solar  system,  lie  other 
vast  systems,  such  as  the  globular  star  clusters,  com- 
posed of  thousands,  possibly  hundreds  of  thousands, 
of  suns;  the  Magellanic  clouds,  which  resemble  de- 
tached portions  of  the  Milky  Way,  and,  probably,  the 
much  discussed  spiral  nebulae,  possible  "island  uni- 
verses" similar  to  our  own. 

We  have  come  far  in  the  past  three  hundred  years 
from  the  conception  of  an  immovable  earth  at  the 
center  of  the  universe  to  this  awe-inspiring  conception 
of  the  universe  that  we  have  today,  which  is  based 
upon  modern  astronomical  discoveries. 

Whatever  may  be  discovered  in  the  future  in  regard 
to  the  form  and  extent  of  the  universe  the  idea  of 
a  fixed  and  immovable  center  either  within  the  solar 
system  or  among  the  stars  beyond  has  gone  from 
the  minds  of  men  at  last. 

Not  more  than  a  generation  ago  a  survival  of  the 


SPIRAL  NEBULA  IN  ANDROMEDA  VIEWED  EDGEWISE 
Taken  with  6o-inch  Reflector  of  the  Mt.  Wilson  Observatory 


MOTIONS  OF  HEAVENLY  BODIES      223 

old  idea  of  a  fixed  center  was  seen  in  the  belief  that 
Alcyone,  in  the  Pleiades  was  a  "central  sun"  about 
which  all  the  stars  revolved.  It  is  now  well  known 
that  the  Pleiades  form  a  moving  star  cluster.  Alcyone 
is  therefore  drifting  slowly  onward  through  the  uni- 
verse and  the  idea  of  a  fixed  and  immovable  center 
to  which  man  may  anchor  his  ideas  is  drifting  away 
also.  There  are,  it  is  true,  local  centers  of  systems, 
such,  for  instance,  as  the  sun  occupies  in  the  solar 
system  or  some  group  of  stars  may  occupy  in  the  stellar 
system  to  which  our  sun  belongs,  yet  as  a  whole  these 
systems  move  on  and  their  centers  with  them.  There 
is  no  evidence  today  that  any  absolutely  immovable 
point  exists  in  the  heavens. 

No  celestial  object  has  been  found  to  be  without 
the  attribute  of  motion,  not  only  motion  onward 
through  the  universe,  but  also  rotational  motion  about 
an  axis  of  the  body.  The  planets  rotate  on  their 
axes  as  well  as  revolve  about  the  sun,  and  the  sun 
also  turns  on  its  axis  as  it  moves  onward  through 
space.  This  rotational  motion  is  also  found  in  the 
nebulae  and  star  clusters  as  well  as  in  the  stars  and 
planets.  No  object  in  the  heavens  is  known  to  be 
without  it.  Even  the  slowly  drifting  Orion  nebula 
possesses  a  rapid  internal  velocity  of  rotation.  There 
is  no  such  thing  as  a  body  absolutely  at  rest  in  the 
universe. 


224      ASTRONOMY  FOR  YOUNG  FOLKS 


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XXVIII 

THE    EVOLUTION     OF    THE     STARS— FROM 
RED  GIANTS  TO  RED  DWARFS 

THE  most  casual  of  star-gazers  is  aware  that  the 
stars  differ  one  from  another  in  color  and  in 
brightness.  There  are  red  stars,  yellow  stars,  white 
stars  and  bluish-white  stars.  There  are  the  brilliant 
stars  of  first  magnitude  such  as  Vega;  Capella  and 
Antares,  and  there  are,  on  the  other  hand,  stars  so 
faint  that  they  can  barely  be  glimpsed  with  the  most 
powerful  telescopes. 

In  general  the  most  brilliant  stars  are  the  nearest 
and  the  faintest  stars  are  the  most  distant,  but  there 
are  many  exceptions  to  the  rule,  since  there  are  stars 
that  appear  faint  even  when  comparatively  near  be- 
cause they  are  small  and  shine  with  a  feeble  light. 
Such  a  star  is  the  faint,  sixth-magnitude  star,  61  Cygni, 
one  of  the  nearest  of  all  the  stars.  Again,  there  are 
stars  in  far-distant  clusters  visible  only  in  powerful 
telescopes  that  in  actual  brightness  exceed  our  own 
sun  several  thousand  times  and  in  volume  several 
million  times.  A  star  the  size  of  the  sun  would  be 
invisible  in  the  most  powerful  telescope  in  existence 
if  it  were  at  the  distance  of  many  stars  in  the  Milky 
Way  or  globular  star  clusters. 

Stars  differ  in  color  because  they  differ  in  tem- 
perature. We  are  all  aware  of  the  fact  that  a  piece 

225 


226      ASTRONOMY  FOR  YOUNG  FOLKS 

of  iron  when  heated  first  glows  a  deep  red,  then  ap- 
pears yellowish  in  color  and  finally  attains  to  white 
heat.  It  is  the  same  among  the  stars.  The  red  stars 
are  the  coolest  of  all  the  stars  and  the  bluish- white 
stars  are  the  hottest  of  all  the  stars,  while  inter- 
mediate between  them  in  temperature  come  the  yellow 
and  the  white  stars. 

Now  as  the  biologist  and  the  geologist  see  in  this 
world  of  ours  signs  of  evolution,  or  gradual  develop- 
ment and  change  from  the  simple  to  the  more  complex 
forms,  and  of  growth  and  decay,  so  the  astronomer 
sees  among  the  stars  signs  of  a  continuous,  progressive 
development  from  one  type  of  star  to  another.  Stars 
share  in  the  general  evolution  that  is  the  law  of  the 
universe,  and  are  born,  reach  the  height  of  their  de- 
velopment, decline  to  old  age  and  die. 

Within  the  past  few  years  important  astronomical 
discoveries  have  been  made  that  show  the  true  order 
of  this  evolution  of  the  stars.  It  was  believed  not  so 
long  ago  that  the  blue-white  helium  stars — the  type 
B  stars  the  astronomers  called  them,  or  the  Orion  stars, 
since  there  are  so  many  stars  of  this  type  in  the  con- 
stellation of  Orion — were  not  only  the  hottest  but  also 
the  youngest  of  the  stars  and  that  they  represented  the 
first  stage  in  the  development  of  a  star  from  a  primi- 
tive gaseous  nebula  such  as  the  Great  Orion  Nebula. 
It  is  now  known  that  these  brilliant,  hot  helium  stars 
represent  the  peak  of  development  of  the  most  mas- 
sive of  all  the  stars  and  not  the  first  stages  in  the 
development. 


EVOLUTION  OF  THE  STARS  227 

A  star,  it  is  now  known,  comes  into  existence  as  a 
giant,  reddish  star  of  enormous  size  and  of  a  density 
only  about  one-thousandth  that  of  the  earth's  atmos- 
phere at  sea-level.  It  is  inconceivably  tenuous  or  rare, 
and  its  temperature  is  comparatively  low,  about  3,000° 
Centigrade  or  less.  It  is  not  evolved  from  the  lumi- 
nous, gaseous  nebulae  because  red  stars  are  never  found 
associated  with  the  gaseous  nebulae,  as  are  the  blue- 
white  stars.  The  origin  of  these  red  giant  stars  is 
uncertain,  but  it  is  possible  that  they  may  be  gradually 
evolved  in  some  manner  from  the  dark  clouds  of  ob- 
scuring matter  or  dark  nebulae  that  exists  so  abun- 
dantly in  the  heavens. 

In  the  next  stage  of  its  development  the  deep-red 
giant  star  increases  in  temperature  as  it  contracts  under 
the  action  of  gravitation  and  its  color  gradually 
changes  from  red  to  yellow.  Its  density  increases 
slightly  and  its  volume  decreases.  It  is  now  a  yellow 
giant  star.  As  the  evolution  progresses  in  the  course 
of  ages  the  star  continues  to  contract,  its  temperature 
increases  greatly  as  does  also  its  density  and  it  con- 
tinues to  decrease  in  volume.  It  is  now  a  brilliant  white 
star,  a  hydrogen  star,  so  called  because  its  spectrum 
is  chiefly  characterized  by  the  lines  of  hydrogen. 

As  the  star  contracts  under  the  gravitation  of  its 
parts  and  increases  in  temperature  and  density  there 
comes  more  and  more  into  play  an  important  factor 
that  has  a  great  effect  upon  its  future  development. 
This  is  light-pressure  or  radiation  pressure  which  acts 
in  opposition  to  gravity  and  exerts  a  strong  outward 


228      ASTRONOMY  FOR  YOUNG  FOLKS 

pressure  upon  matter  within  the  depths  of  the  star, 
tending  to  push  it  outward  from  the  center  where 
the  temperature  is  greatest  and  the  light  is  most  intense. 
It  is  a  most  interesting  fact  that  if  the  mass  of  a  star,  that 
is  the  quantity  of  matter  that  it  contains,  exceeds  a  certain 
value  the  pressure  of  light  or  radiation  within  it  over- 
balances the  gravitational  attraction  that  draws  matter 
towards  its  center  and  the  star  disintegrates  or  ceases 
to  exist  as  a  star.  This  accounts  for  the  fact  that  the 
stars  differ  very  little  among  themselves  in  the  quan- 
tity of  matter  that  they  contain,  that  is,  in  their  masses, 
though  they  may  differ  enormously  in  size.  Stars  that 
exceed  a  certain  mass  will  become  unstable  and  this 
may  account  for  the  association  of  luminous  nebulae 
with  the  hottest  of  all  stars,  the  nebulae  possibly  being 
puffed  off  from  the  surfaces  of  these  stars  under  the 
action  of  radiation  pressure. 

After  a  star  has  reached  the  height  of  its  develop- 
ment as  a  bluish-white  helium  star  with  a  temperature 
of  something  like  10,000°  Centigrade  and  a  density 
about  one-tenth  that  of  the  sun,  it  begins  to  lose  heat 
and  to  cool  gradually  though  it  continues  to  contract 
and  increase  in  density. 

It  is  now  on  the  descending  scale  of  evolution  and  is 
to  be  counted  among  the  dwarfs  instead  of  the  giants. 
A  brilliant  blue-white  helium  or  Orion  star  is  about 
one  hundred  times  more  luminous  than  the  sun,  and  its 
diameter  is  about  ten  times  that  of  the  sun. 

Our  own  sun,  we  find,  is  on  the  descending  scale  of 
stellar  evolution.  It  is  a  yellow  dwarf  star  of  tempera- 


EVOLUTION  OF  THE  STARS  229 

ture  about  6,000°  Centigrade  and  density  one  and  one- 
fourth  that  of  water,  which  is  probably  about  as  great 
a  density  as  is  attained  by  any  star  since  even  the  non- 
luminous  planets  Jupiter  and  Saturn  have  lower  densi- 
ties than  the  sun. 

The  last  stage  in  the  development  of  a  star  is  rep- 
resented by  the  dwarf  red  star  of  high  density  and 
low  temperature.  The  diameter  of  the  dwarf  red  star 
probably  averages  about  five  hundred  thousand  miles 
and  its  temperature  is  3,000°  Centigrade  or  less.  After 
this  we  have  the  extinct  stars,  similar  possibly  to  our 
planet  Jupiter,  though  considerably  larger,  with  a  dense 
gaseous  atmosphere  and  a  certain  degree  of  internal 
heat. 

We  have  traced  the  evolution  of  a  star  from  a  red 
giant  to  a  red  dwarf  through  the  intermediate  stages 
from  yellow  giant  to  a  giant  helium  star  with  increas- 
ing temperature  and  thence  to  yellow  dwarf  and  red 
dwarf  as  the  temperature  decreases.  Only  the  most 
massive  stars  pass  through  this  entire  chain  of  evolu- 
tion. Stars  of  small  mass  never  attain  to  the  splendor 
of  brilliant  blue-white  helium  stars,  but  begin  to  de- 
crease in  temperature  and  brightness  before  this  stage 
is  reached. 

The  time  required  for  the  evolution  of  a  star  from 
red  giant  to  red  dwarf  is  not  known,  but  it  must  be  very 
great.  The  age  of  the  earth,  which  is  probably  equal 
to  that  of  the  solar  system,  is  estimated  as  something 
like  one  thousand  million  years.  It  is  probable  that 
the  average  life  of  a  star  far  exceeds  this  limit. 


XXIX 

DOUBLE  AND  MULTIPLE  STARS 

THE  plan  of  the  solar  system  whidi  consists  of  a 
central  sun  encircled  by  satellites  that  are  far  in- 
ferior to  their  luminary  in  size,  and  that  move  about  it 
in  orbits  that  are  almost  perfect  circles,  is  not  the  only, 
nor  possibly,  even  the  most  general  one  in  the  universe. 
Sweeping  the  heavens  with  powerful  telescopes  one  is 
astonished  to  find  that  myriads  of  stars  can  be  sepa- 
rated into  two  or  more  physically  connected  suns  that 
are  often,  moreover,  of  exquisitely  tinted  and  contrast- 
ing shades.  Green  and  red,  orange  and  blue,  white 
and  golden  or  white  and  blue  pairs  exist  in  profusion, 
and  strange  to  say  there  are  well-authenticated  in- 
stances of  color  changes  taking  place  temporarily  within 
the  same  system.  A  pair  of  white  stars  has  been  known 
to  change  within  a  few  decades,  first  to  golden  yellow 
and  bluish  green  and  then  to  orange  and  green.  The 
famous  pair  catalogued  as  "95  Herculis"  was  noted  to 
change  from  green  and  red  to  a  palish  yellow  and  back 
to  the  original  strongly  contrasting  hues  within  the 
course  of  a  single  year,  while  at  another  time  they  ap- 
peared to  be  a  perfectly  white  pair.  At  the  present 
time  both  of  these  stars  are  decidedly  yellowish  in  color. 
Such  changes  in  hue  are  probably  due  to  temporary  dis- 

230 


DOUBLE  AND  MULTIPLE  STARS       231 

turbances  in  the  atmospheres  of  the  stars,  possibly  of 
an  electrical  nature  or  to  sudden  or  unusual  outbursts  of 
activity,  concerning  the  origin  of  which  we  are  as  much 
in  doubt  as  we  are  of  the  cause  of  the  sunspot  cycle 
and  periodic  variation  in  the  intensity  of  radiation  of 
our  own  sun.  Temporary  changes  in  the  color  of  the 
components  of  double  star  systems  sometimes  take  place 
when  the  two  stars  approach  their  "periastron"  or  point 
of  nearest  approach.  Owing  to  the  great  eccentricity 
of  the  orbits  of  double  stars,  such  stars  are  anywhere 
from  twice  to  nineteen  times  as  near  to  each  other  at 
periastron  as  they  are  at  "apastron,"  or  point  of  great- 
est departure.  Such  great  changes  in  the  relative  dis- 
tances of  two  physically  connected  suns  would  produce 
marked  changes  in  the  intensity  of  the  tides  raised  upon 
each  of  them  by  their  mutual  gravitational  attraction 
and  unusual  outbursts  of  gases  or  electrical  excitement 
in  the  atmospheres  of  the  stars  might  cause  very  notice- 
able changes  in  the  color  of  these  stars  as  they  drew 
nearer  to  each  other,  which  would  subside  as  they  re- 
ceded toward  apastron. 

In  addition  to  "visual"  double  or  multiple  stars,  there 
exists  a  very  extensive  class  of  stars  known  as  "spectro- 
scopic  binaries,"  in  which  the  two  components  are  so 
close  to  each  other  that  even  the  most  powerful  tele- 
scopes cannot  divide  them.  It  is  only  from  the  shifting 
of  the  lines  of  their  overlapping  spectra,  caused  by  their 
alternate  motion  toward  and  from  the  earth  as  they 
revolve  about  their  common  center  of  gravity,  that  their 
duplex  nature  is  revealed  to  us; 


232      ASTRONOMY  FOR  YOUNG  FOLKS 

In  some  instances  one  member  of  the  system  is  so 
faint  that  its  spectrum  is  not  visible  and  its  presence 
is  disclosed  only  by  the  shifting  of  the  lines  of  the 
bright  star. 

According  to  Doppler's  Law,  when  a  star  is  approach- 
ing the  earth  the  lines  of  its  spectrum  shift  toward  the 
blue  end  of  the  spectrum,  and  when  the  star  is  reced- 
ing from  the  earth  the  lines  are  shifted  toward  the  red 
end  of  the  spectrum.  The  amount  of  this  shift  can  be 
very  accurately  measured,  and  gives  the  relative  veloci- 
ties of  the  stars  in  their  orbits  directly  in  miles  per  sec- 
ond. Knowing  in  addition,  by  observation,  the  period 
of  mutual  revolution  of  the  stars,  it  is  possible  to  find 
the  dimensions  of  these  spectroscopic  binary  systems 
compared  to  our  own  solar  system,  and  also  the  masses 
of  the  stars  compared  to  the  mass  of  our  own  sun.  If 
the  spectrum  of  the  fainter  star  is  not  visible,  only  the 
velocity  of  the  brighter  star  with  respect  to  the  center 
of  gravity  of  the  system  can  be  found  and  the  mass 
found  for  the  system  comes  out  too  small.  In  such 
cases  we  can  obtain  only  a  lower  limit  for  the  mass 
of  the  system.  Then,  too,  it  must  be  remembered  that 
these  systems  of  stars  lie  at  all  angles  with  reference 
to  our  line  of  sight,  and  so  we  rarely  see  the  orbits  in 
their  true  form.  The  measured  velocities  are  as  a  re- 
sult smaller  than  the  true  velocities,  and  on  the  average 
amount  to  only  sixty  per  cent,  of  the  true  orbital  veloci- 
ties. The  calculated  masses. of  spectroscopic  binary  stars 
are,  therefore,  in  general  only  about  sixty  per  cent,  of 
the  true  masses.  It  hag  be.en  found  from  calculating 


DOUBLE  AND  MULTIPLE  STARS       233 

the  masses  of  a  number  of  binary  systems,  that  the  com- 
bined masses  of  the  stars  in  these  systems  do  not  differ 
very  greatly  among  themselves,  nor  as  compared  to  our 
own  sun,  though  in  light-giving  power  these  stars  may 
differ  hundreds,  thousands,  even  millions  of  times.  For 
instance,  there  are  stars  that  give  only  one  ten-thou- 
sandth part  of  the  light  of  our  own  sun,  and  other 
stars  that  give  ten  thousand  times  as  much  light  as  the 
sun.  Moreover,  there  are  many  instances  of  physi- 
cally connected  stars  differing  thousands  of  times  in 
luminosity,  though  in  mass,  or  quantity  of  matter  found 
in  the  stars,  they  differ  only  two  or  three  times.  Why 
this  is  so  remains  one  of  the  great  mysteries  of  the 
heavens,  and  makes  it  extremely  difficult  to  give  any 
satisfactory  theory  of  the  origin  of  double-star  systems. 
It  has  never  been  explained  satisfactorily  why  of  two 
suns  physically  connected  and,  therefore,  presumably 
originating  at  the  same  time,  one  should  be  radiating 
with  the  greatest  intensity,  while  the  other  is  practically 
an  extinct  sun,  in  spite  of  the  fact  that  the  quantity 
of  matter  in  the  two  bodies  differs  but  slightly. 
In  a  few  systems  the  plane  in  which  the  stars  revolve 
passes  so  nearly  through  the  earth  that  the  two  stars 
temporarily  eclipse  one  another  during  each  revolution. 
Such  systems  are  called  eclipsing  binaries.  To  such 
a  system  belongs  the  famous  Algol.  Its  light  waxes 
and  wanes  periodically  with  the  greatest  punctuality  in 
a  period  of  2d  2011  48.9ra,  owing  to  its  temporary  eclipse 
by  a  very  large  but  extremely  faint  attendant  sun.  The 
diameter  of  the  faint  star  is  slightly  greater  than  the 


234      ASTRONOMY  FOR  YOUNG  FOLKS 

diameter  of  the  bright  star  which  is  about  one  million 
miles  in  extent.  The  distance  between  the  centers  of 
the  stars  is  only  about  three  million  miles,  which  brings 
their  surfaces  within  two  million  miles  of  each  other. 
The  masses  of  the  two  stars  are  in  the  ratio  of  two 
to  one,  the  brighter  and  more  massive  star  being  about 
half  as  heavy  as  our  own  sun,  though  its  density  is 
only  about  two-tenths  that  of  the  sun.  The  density  of 
the  fainter  star  is  still  less,  being  only  about  half  that 
of  the  brighter  star.  Very  low  density  of  both  compo- 
nents and  extreme  faintness  of  one  member  compared 
to  the  other,  appears  to  be  a  very  general  characteristic 
of  closely  associated  eclipsing  and  spectroscopic  binary 
stars.  Among  the  extremely  hot  and  brilliant  helium 
and  hydrogen  stars,  spectroscopic  binaries  exist  in  great 
numbers.  In  fact,  among  these  types  there  appear  to  be 
as  many  binary  and  multiple  systems  as  there  are  sys- 
tems of  isolated  suns.  Sometimes  these  close  binary 
stars  are  egg-shaped  or  oval  and  revolve  rapidly  almost 
in  contact  about  their  common  center  of  gravity.  In- 
habitants of  satellites  of  such  a  system  would  see  in 
their  heavens  the,  to  us,  strange  and  startling  phenom- 
enon of  two  suns,  each  equal  to  our  own  or  even  great- 
er in  size,  whirling  rapidly  about  each  other  and  sepa- 
rated by  a  space  comparable  in  extent  to  their  own 
diameters.  Eclipses  in  such  a  system  would  be  of 
daily  occurrence,  and,  if  one  star  were  dark,  would  pro- 
duce for  the  satellite  world  the  same  effect  of  alternate 
day  and  night  that  results  from  axial  rotation  of  a 
satellite.  The  two  hemispheres  of  the  faint  compan- 


DOUBLE  AND  MULTIPLE  STARS       235 

ion  sun  would  be  very  unequally  illuminated  owing  to 
the  fact  that  the  side  turned  toward  the  brilliant  sun 
would  always  reflect  its  neighbor's  brightness  in  ad- 
dition to  shining  with  its  own  comparatively  feeble  in- 
herent light,  while  the  opposite  hemisphere  would  shine 
only  by  its  own  dim  light,  and  would,  therefore,  be  in 
comparative  darkness. 

The  spectroscopic  binaries  generally  revolve  closely 
and  rapidly  about  their  common  center  of  gravity; 
there  are  to  be  found,  on  the  other  hand,  among  the 
wider  visual  doubles,  many  systems  wherein  the  com- 
ponents are  separated  by  distances  comparable  to  the 
distances  of  the  outer  planets,  Saturn,  Uranus  and  Nep- 
tune, from  the  sun.  It  is  evident  that  the  individual 
stars  of  such  binary  systems  could  not  possibly  be  en- 
circled by  any  such  extensive  system  of  satellites  as  at- 
tends our  own  sun,  though  satellites  such  as  our  own 
planet  Earth,  or  the  inferior  planet's  Mercury  and 
Venus,  might  conceivably  encircle  the  individual  com- 
ponents of  such  binary  systems  at  distances  not  greater 
than  that  of  the  earth  from  the  sun.  No  planet  could 
safely  exist  at  a  much  greater  distance  from  one  of 
these  suns  without  being  subject  to  most  dangerous 
perturbations  and  disruptive  tidal  forces  arising  from 
the  vicinity  of  the  second  sun.  Granted  that  planets 
might  encircle  one  of  these  suns  at  a  distance  approx- 
imating that  of  Venus  or  our  own  planet  from  the  sun, 
the  inhabitants  of  such  worlds  would  behold  the  strange 
phenomenon  of  two  suns  in  the  heavens,  not  almost  in 
contact  as  in  spectroscopic  binary  systems,  but  at  one 


236      ASTRONOMY  FOR  YOUNG  FOLKS 

time  comparatively  near  and  again  in  opposite  portions 
of  the  heavens  as  is  the  case  with  the  sun  and  moon 
in  our  own  heavens.  As  the  planet  advanced  in  its  orbit 
about  the  ruling  sun,  the  secondary  sun  would  be  visible 
at  first  by  day  and  again  by  night.  If  the  two  suns 
were  of  contrasting  hues,  as,  for  instance,  green  and 
red,  there  might  appear  in  the  nearby  heavens  at  a  dis- 
tance of  one  hundred  million  miles  or  so  a  magnificent 
sun  of  deep  reddish  hue,  equal  to  or  surpassing  our 
own  in  splendor,  while  in  a  far  distant  part  of  the  sky, 
at  a  distance  as  great  as  that  which  separates  us  from 
the  planet  Saturn,  might  appear  a  rival  sun  of  greenish 
hue,  smaller  and  fainter,  but  nevertheless,  hot  and  ex- 
tremely brilliant  and  capable  of  exerting  through  its 
great  gravitational  attraction  a  most  disturbing  effect 
upon  the  motion  of  the  planet  of  its  neighbor.  At 
times  the  rays  of  the  two  suns,  red  and  green,  would 
combine  to  produce  a  day  characterized  by  terrific  heat 
and  intense  illumination.  Again  the  green  orb  would 
rise  in  the  east  as  the  red  sun  set  in  the  west  and  night 
would  be  turned  into  a  weird,  dimly-lighted  day  by  the 
greenish  rays  of  the  secondary  sun.  Compared  to  the 
wonders  and  beauties  of  the  heavens  in  such  a  sys- 
tem, our  own  well-regulated  and  orderly  planet  family, 
undisturbed  by  the  exciting  proximity  of  a  rival  sun, 
seems  to  pale  into  insignificance.  Yet  we  have  every 
good  reason  to  be  content  with  the  ordering  of  affairs 
within  our  own  solar  system,  and  to  feel  relief  rather 
than  regret  at  the  absence  of  a  secondary  sun.  In  a 
planet  world  revolving  about  one  member  of  a  double 


DOUBLE  AND  MULTIPLE  STARS        237 

star  system,  we  may  imagine  the  dread  rather  than  plea- 
sure with  which  the  periodic  near-approach  of  a  rival 
sun  would  be  hailed,  and  even  the  possible  hurried  mi- 
gration from  exposed  to  sheltered  portions  of  the  plane- 
tary world  to  escape  the  rapidly  increasing  heat  and 
intensity  of  light  from  the  approaching  sun.  In  such 
systems  the  coming  and  going  of  the  seasons  might 
indeed  be  a  matter  of  life  and  death  to  the  inhabi- 
tants of  satellite  worlds! 

Within  our  solar  system  the  masses  of  the  planets  are 
practically  negligible  compared  to  the  mass  of  the  sun, 
and  it  is  for  this  reason  that  they  appear  to  revolve 
about  the  center  of  the  sun.  As  a  matter  of  fact,  no 
body  in  the  universe  revolves  about  the  exact  geometri- 
cal center  of  another  body,  but  two  mutually  attracting 
bodies  revolve  in  orbits  about  their  common  center  of 
gravity,  which  always  lies  between  the  two  bodies  on 
the  line  connecting  them  and  at  a  distance  from  each 
of  them  that  is  in  inverse  proportion  to  the  mass  of 
the  body.  The  moon  does  not  revolve  about  the  center 
of  the  earth,  but  about  the  center  of  gravity  of  the  earth 
and  moon,  which  lies  on  the  line  connecting  the  two 
bodies  and  at  a  distance  from  the  earth's  center  that 
is  one  eighty-first  of  the  distance  from  the  center  of  the 
earth  to  the  center  of  the  moon,  since  this  represents  the 
ratio  of  the  masses  of  the  two  bodies.  This  center  of 
gravity  of  the  earth  and  moon,  lies,  then,  about  two 
thousand  miles  from  the  earth's  center,  and  about  this 
point  both  earth  and  moon  trace  out  orbits  of  revolution 
that  are  identical  in  form  and  differ  only  in  size.  In 


238      ASTRONOMY  FOR  YOUNG  FOLKS 

the  same  way  each  of  the  planets  of  the  solar  system 
revolves  about  the  center  of  gravity  of  itself  and  the 
sun,  but  the  mass  of  the  sun  is  so  far  in  excess  of  the 
combined  masses  of  all  the  planets  that  we  may  con- 
sider, for  all  practical  purposes,  that  the  planets  revolve 
about  the  sun's  center,  the  center  of  gravity  of  the 
system  being  within  the  sun,  just  as  the  center  of 
gravity  of  the  earth  and  moon  is  within  the  earth. 

Prof.  T.  J.  J.  See  found  from  the  investigation 
of  forty  binary  star  orbits  that  the  average  eccentricity 
of  double  star  orbits  is  twelve  times  as  great  as  the 
average  eccentricity  of  a  planetary  orbit,  and  that  the 
masses  of  the  component  suns  never  differ  very 
greatly.  The  center  of  gravity  of  a  binary  system, 
therefore,  lies  at  a  great  distance  from  the  centers 
of  the  stars,  and  about  this  point,  as  a  focus,  the  stars 
move  in  orbits  that  are  exactly  similar  in  form  but 
differ  in  size  in  inverse  proportion  to  the  ratio  of 
the  masses.  Since  the  orbits  of  binaries  are,  moreover, 
very  highly  eccentric,  the  two  suns  are,  as  we  have 
said,  anywhere  from  two  to  nineteen  times  nearer  to 
each  other  at  periastron  than  they  are  at  "apastron." 

We  have  spoken  so  far  only  of  systems  of  two 
associated  suns,  but  many  systems  exist  in  which  three 
or  more  sun-like  bodies  are  in  revolution  about  a 
common  center  of  gravity.  Frequently  two  fairly 
close  suns  are  in  revolution  about  a  common  center 
of  gravity,  in  a  period,  say,  of  fifty  or  sixty  years, 
while  a  third  sun  revolves  at  a  comparatively  great 
distance  about  the  center  of  gravity  of  itself  and 


DOUBLE  AND  MULTIPLE  STARS       239 

the  first  pair  in  a  period  of  several  hundred  years. 
Or  possibly  the  third  sun  also  possesses  a  close  at- 
tendant and  the  two  pairs  revolve  in  a  period  of  great 
length  about  a  common  center  of  gravity. 

Such,  for  instance,  are  the  systems  of  Zeta  Cancri 
and  Epsilon  Lyrae.  In  the  former  system  the  closer 
components  revolve  rapidly  about  their  center  of  gravity 
in  a  period  of  about  sixty  years,  while  the  remote  com- 
panion shows  irregularities  in  its  motion  that  indicate 
that  it  is  revolving  about  a  dark  body  in  a  period  of 
seventeen  and  a  half  years,  while  the  two  together  are 
revolving  very  slowly  in  a  period  of  six  or  seven  centuries, 
about  a  common  center  of  gravity  with  the  first  pair 
in  a  retrograde  direction. 

The  wider  pair  of  Epsilon  Lyrae  is  a  naked-eye 
double  for  it  can  be  seen  as  a  double  star  by  a  keen 
eye,  while  even  a  three-inch  telescope  will  separate 
each  of  the  components  into  a  double  star.  So  ex- 
tensive is  this  system  that  the  periods  of  revolution  of 
the  closer  components  occupy  several  centuries,  one 
pair  appearing  to  revolve  about  twice  as  rapidly  as 
the  other,  while  the  period  of  revolution  of  the  two 
pairs  about  a  common  center  is  probably  a  matter  of 
thousands  of  years.  The  gap  that  separates  the  two 
pairs  may  be  so  great  that  light  requires  months  to 
cross  it. 

These  multiple  systems  are  by  no  means  exceptional. 
They  are  to  be  found  in  profusion  among  the  brilliant 
Orion  stars.  They  have  been  referred  to  as  "knots"  of 
stars  and  it  has  been  suggested  that  they  may  have 


240      ASTRONOMY  FOR  YOUNG  FOLKS 

originated  as  local  condensations  in  one  vast  nebulous 
tract.  A  system  of  only  two  components  appears  to  be 
the  exception  rather  than  the  rule,  groups  of  several 
connected  suns  being  more  numerous  than  single  pairs. 

In  all  of  these  double  and  multiple  systems  there 
exists  the  possibility  of  minute  satellites,  such  as  our 
own  earth,  in  attendance  upon  some  one  component  of 
the  system.  Such  tiny  bodies  shining  only  by  reflected 
light  from  a  nearby  brilliant  sun  would  be  hopelessly 
invisible  in  the  most  powerful  telescope. 

We  can  only  assume  that  it  is  far  more  reasonable 
to  believe  in  than  to  disprove  the  existence  of  such 
satellites. 

Our  own  solar  system,  then,  represents  neither  in  its 
mechanical  nor  physical  features,  the  only  possibilities 
for  the  maintenance  of  life;  it  can  neither  be  con- 
sidered a  unique  form,  nor  even  the  most  generally 
prevalent  form  in  the  universe. 


XXX 

ASTRONOMICAL  DISTANCES 

THE  grandeur  of  the  scale  upon  which  the  visible 
universe  is  fashioned  lies  almost  beyond  human 
comprehension.  In  measuring  the  vast  extent  of  our 
own  solar  system,  which  is  but  a  single  unit  in  the 
system  of  the  stars,  we  may  have  recourse  to  some 
earthly  standard  of  measurement,  such  as  the  mile. 
But  when  we  desire  to  express  in  terms  of  units  that 
can  be  grasped  by  our  imagination,  the  distances  of  the 
stars  that  lie  far,  far  beyond,  we  find  that  all  ordinary 
standards  of  measurement  become  utterly  inadequate 
for  our  purpose.  In  the  measurement  of  celestial  dis- 
tances within  the  solar  system  the  unit  employed  is 
either  the  familiar  mile  or  kilometer  or  the  "astro- 
nomical unit,"  which  is  the  mean  distance  from  the 
earth  to  the  sun  (ninety-two  million  nine  hundred 
thousand  miles  in  round  numbers). 

In  the  measurement  of  distances  beyond  the  solar 
system  the  unit  employed  is  either  the  light-year  or 
more  recently  the  parsec,  which  is  rapidly  replacing  the 
light-year  among  astronomers.  A  "light-year"  is  the 
distance  that  light,  with  its  finite  but  almost  unimagin- 
able velocity  of  one  hundred  and  eighty-six  thousand 
miles  per  second,  travels  in  a  year.  It  is  equal  in  round 

241 


242      ASTRONOMY  FOR  YOUNG  FOLKS 

numbers  to  sixty-three  thousand  times  the  distance  from 
the  earth  to  the  sun  or  approximately  six  thousand 
billions  of  miles.  The  parsec  is  equal  to  three  and 
twenty-six  hundredths  (3.26)  light-years,  and  it  is 
approximately  two  hundred  thousand  times  the  distance 
from  the  earth  to  the  sun.  It  is  "the  distance  of  a  star 
with  the  parallax  of  a  second,"  a  fact  which  its  name, 
parsec,  conveys  to  us.  In  other  words,  at  the  distance 
of  one  parsec  the  distance  from  the  earth  to  the  sun, 
"the  astronomical  unit/'  would  subtend  an  angle  equal 
to  one  second  of  an  arc.  This  angle  is  spoken  of  as  the 
parallax  of  the  star.  The  larger  the  parallax,  that  is, 
the  larger  the  angle  the  astronomical  unit  or  radius 
of  the  earth's  orbit  subtends,  viewed  from  the  star,  the 
nearer  the  star  is  to  us.  The  fact  that  there  is  no 
known  star  within  one  parsec,  or  three  and  twenty-six 
hundredths  light-years,  of  the  sun  shows  the  immensity 
of  the  scale  of  the  universe  of  stars. 

Before  considering  the  distances  of  the  stars  and  the 
extent  of  the  sidereal  system  of  which  our  sun  and  his 
satellites  form  a  part,  let  us  undertake  to  express  the 
distance  of  the  sun,  moon  and  planets  from  the  earth 
and  the  extent  of  the  solar  system  in  terms  with  which 
we  are  familiar. 

The  nearest  to  the  earth  of  all  celestial  bodies  is  its 
satellite,  the  moon.  So  near  is  the  moon  that  if  we 
should  make  on  some  great  plain  a  model  of  the  solar 
system  in  which  the  astronomical  unit,  the  distance 
from  earth  to  sun,  would  be  four  hundred  feet,  the 
distance  between  the  earth  and  moon  would  be  only 


ASTRONOMICAL  DISTANCES  243 

one  foot.  On  the  same  scale  the  most  distant  planet 
Neptune  would  be  two  and  one-quarter  miles  away. 

Granted  that  it  were  possible  to  escape  the  earth's 
gravitational  bonds  and  to  travel  by  our  swiftest  means 
of  conveyance,  the  airplane,  through  interplanetary 
space,  let  us  consider  how  long  it  would  take  us  to 
reach  the  moon,  sun  and  planets  if  our  speed  were 
maintained  at  a  uniform  rate  of  two  hundred  miles  an 
hour.  An  airplane  traveling  at  this  rate  would  circum- 
navigate the  earth  in  a  little  over  five  days  and  would 
reach  the  moon  in  seven  weeks.  A  trip  to  the  sun, 
however,  would  take  fifty-three  years. 

After  traveling  for  fourteen  and  a  fraction  years 
we  would  pass  the  orbit  of  Venus  and  eighteen  years 
later  the  orbit  of  Mercury.  If  we  preferred  to  travel 
outward  from  the  earth  in  the  direction  of  Mars  and 
the  outer  planets  instead  of  toward  the  sun,  more  than 
twenty- seven  years  would  elapse  before  we  would  reach 
the  orbit  of  Mars.  An  airplane  journey  to  Jupiter 
would  be  a  matter  of  more  than  two  hundred  years, 
to  Saturn  four  hundred  and  fifty  years,  to  Uranus 
nearly  one  thousand  years,  and  to  Neptune,  about  one 
thousand  five  hundred  years.  To  cross  the  solar  system 
on  the  diameter  of  Neptune's  orbit  in  an  airplane, 
traveling  day  and  night  without  stopping  at  the  rate  of 
200  miles  per  hour  would  take  more  than  three  thou- 
sand years.  The  sun's  attraction  reaches  far  beyond 
Neptune's  orbit,  however.  There  are  comets  belonging 
to  the  solar  system  compelled  by  the  sun's  attraction  to 
accompany  him  on  his  travels  through  space  that  return 


244      ASTRONOMY  FOR  YOUNG  FOLKS 

periodically  to  the  immediate  vicinity  of  the  sun  from 
regions  far  beyond  the  orbit  of  Neptune  and  there  is 
also  the  possibility  that  one  or  more  undiscovered 
planets  may  travel  around  the  sun  in  orbits  far  exterior 
to  Neptune's  orbit. 

Measured  in  terms  of  familiar  units,  such  as  are 
employed  for  the  measurement  of  distances  on  our 
own  planet,  the  extent  of  the  solar  system  is  tremen- 
dously great.  Viewed  from  Neptune,  'the  sun  is 
so  far  away  that  it  presents  no  appreciable  disk.  It 
is  in  this  sense  star-like  to  the  Neptunians,  but  at  the 
distance  of  Neptune  the  stars  appear  no  more  brilliant 
arid  no  nearer  than  they  do  to  us. 

To  Neptune  the  sun,  though  star-like  in  form,  sup- 
plies a  very  appreciable  quantity  of  light  and  heat 
(one  nine-hundredth  of  the  amount  the  earth  receives) 
while  the  amount  of  light  and  heat  that  Neptune 
receives  from  the  nearest  stars  is  entirely  inappreciable. 
When  our  airplane  reaches  Neptune  after  a  journey 
of  one  thousand  five  hundred  years,  it  is,  as  it  were, 
just  clearing  the  ground  for  its  flight  to  the  stars. 
To  cover  the  intervening  space  to  the  nearest  star, 
traveled  by  light  in  four  and  a  third  years,  an  airplane 
would  need  fourteen  and  a  half  million  years.  In 
that  time  the  solar  system  itself  would  be  in  some 
far  distant  part  of  the  universe,  since  it  is  speeding 
onward  through  space  at  the  rate  of  twelve  miles  a 
second  or  about  four  astronomical  units  a  year. 

Changing  new  our  unit  of  measurement  that  we 
may  express  interstellar  distances  in  comprehensible 


ASTRONOMICAL  DISTANCES  245 

numbers,  we  prepare  to  travel   from  the  earth  to  the 
stars  with  the  velocity  of  light. 

With  this  velocity,  one  hundred  and  eighty-six 
thousand  miles  per  second,  we  circumnavigate  our 
globe  in  one-seventh  of  a  second,  reach  the  moon  in 
one  and  a  fourth  seconds  and  the  sun  in  eight  minutes. 
In  a  little  over  four  hours  we  pass  the  orbit  of 
Neptune  and  are  started  on  our  journey  to  the  stars, 
penetrating  further  and  further  into  interstellar  space. 
For  a  year  we  travel  and  reach  not  a  single  star 
though  we  are  speeding  ever  onward  with  the  velocity 
of  light.  We  have  now  covered  the  distance  of  one 
light-year,  which  means  that  the  waves  of  light  from 
the  sun  we  have  left  behind  must  travel  for  a  year 
before  they  reach  us.  We  continue  our  journey  and 
find  ourselves  next  at  a  distance  of  one  parsec  from 
the  sun.  We  have  traveled  a  distance  of  approxi- 
mately three  and  a  quarter  light-years,  and  were  it 
possible  to  see  the  earth  as  well  as  the  sun  at  this  dis- 
tance, the  two  would  appear  to  be  but  one  second  of 
arc  apart,  a  distance  that  requires  the  most  careful 
adjustment  and  manipulation  of  the  telescope  to  mea- 
sure accurately.  We  are  still  one  light-year  distant 
from  Alpha  Centauri,  the  nearest  of  the  bright  stars. 
A  few  of  the  stars  will  now  appear  somewhat  brighter 
than  they  appeared  to  us  on  earth,  but  the  majority 
of  the  stars  appear  just  as  we  see  them  here  and  the 
forms  of  the  constellations  remain  practically  un- 
changed in  appearance,  for  we  are  only  beginning  our 
journey  through  the  sidereal  universe  and  our  posi- 


246      ASTRONOMY  FOR  YOUNG  FOLKS 

tion  in  it  has  only  shifted  by  a  very  slight  amount. 
If  we  should  continue  our  journey  to  the  immediate 
vicinity  of  Alpha  Centauri,  we  would  find  that  it  is 
not  like  our  own  sun,  a  single  star,  but  is  a  binary 
star  consisting  of  two  suns  in  revolution  around 
their  common  center  of  gravity.  The  distance  of  this 
binary  system  from  the  solar  system  has  been  mea- 
sured with  considerable  accuracy  and  is  known  to 
be  four  and  a  third  light-years.  Though  there  may 
be  a  few  faint  stars  or  non-luminous  stars  nearer  to 
us  than  Alpha  Centauri,  this  star  has  long  held  the 
distinction  of  being  the  nearest  of  the  stars.  As  the 
sun  continues  his  journey  through  the  universe  the 
two  stars,  Alpha  Centauri  and  our  sun,  will  finally 
draw  away  from  each  other  after  many  ages  have 
passed  and  some  other  sun  of  space  will  be  our  nearest 
star.  The  distances  that  separate  the  stars  from  each 
other  probably  average  as  great  as  the  distance  from 
the  sun  to  Alpha  Centauri.  Within  a  sphere  whose 
center  is  at  the  earth  and  whose  radius  is  five  parsecs, 
or  about  sixteen  light-years,  there  are  only  about 
twenty  known  stars.  There  is,  therefore,  small  chance 
of  collision  among  bodies  that  are  so  small  in  pro- 
portion to  the  tremendous  intervals  of  space  that 
separate  them  from  each  other.  There  is  ample  room 
for  the  individual  stars  to  pursue  their  journey  through 
space  without  interfering  with  each  other's  motion 
so  long  as  they  are  as  widely  scattered  as  they  appear 
to  be  in  this  portion  of  the  universe.  The  fact  that 
our  own  sun  has  continued  its  journey  through  the 


ASTRONOMICAL  DISTANCES  247 

universe  for  some  hundreds  of  millions  of  years  with- 
out any  catastrophe  such  as  would  result  from  closely 
approaching  or  colliding  with  another  sun  of  space 
shows  how  enormous  is  the  scale  upon  which  our 
sidereal  system  is  fashioned. 

Stars  that  are  ten,  fifty  or  even  one  hundred  light- 
years  from  the  earth  are  our  nearest  neighbors  in 
space.  They  are  the  stars  that  show  a  slight  displace- 
ment in  the  heavens  or  measurable  parallax,  viewed 
from  opposite  sides  of  the  earth's  orbit.  There  are 
probably  a  thousand  stars  among  the  hundreds  of 
millions  of  stars  within  reach  of  the  greatest  telescopes 
whose  distances  have  been  determined  in  light-years 
by  direct  measurement  of  their  displacement  in  the 
heavens  resulting  from  the  change  of  position  of  the 
earth  in  its  orbit.  The  most  distant  of  the  stars  are 
apparently  immovable  in  the  heavens  showing  neither 
the  effect  of  the  sun's  motion  or  their  own  motion 
through  space.  Methods  for  finding  the  distances  of 
many  far  remote  stars  and  star-clusters  have  been 
devised,  however,  and  some  comparatively  recent  in- 
vestigations have  given  results  for  the  distances  of 
these  objects  indicating  that  the  diameter  of  the  system 
of  stars  to  which  our  sun  belongs  is  approximately 
three  hundred  thousand  light-years.  It  is  difficult 
to  grasp  the  full  significance  of  this  fact.  It  means 
that  hundreds  of  millions  of  the  suns  of  space  throng 
the  visible  universe  at  distances  from  us  and  from 
each  other  running  into  hundreds,  thousands  and  even 
hundreds  of  thousands  of  light-years.  The  light  waves 


248      ASTRONOMY  FOR  YOUNG  FOLKS 

from  some  tiny  object  that  we  view  today  in  one 
of  our  great  reflectors  may  have  started  on  their 
journey  through  space  over  one  hundred  thousand  years 
ago  when  men  of  the  Old  Stone  Age  inhabited  our 
planet  earth! 

Astronomers  have  found  as  a  result  of  their  investi- 
gations that  the  sidereal  system  to  which  our  solar 
system  belongs  is  in  the  form  of  a  flattened  spheroid 
with  its  longest  axis  in  the  plane  of  the  Milky  Way. 
The  extent  of  this  star  system  composed  of  hundreds 
of  millions  of  individual  suns  in  addition  to  nebulae 
and  clusters  is  probably  something  like  three  hundred 
thousand  light-years  along  its  longest  axis,  while  globu- 
lar star  clusters  lying  above  and  below  its  central  plane 
are  estimated  to  be  at  distances  from  it  ranging  from 
ten  thousand  to  two  hundred  thousand  light-years. 
This  entire  organized  system  is  our  sidereal  universe. 
Space  beyond  is  unexplored.  The  globular  star  clusters 
are  among  the  most  distant  celestial  objects  so  far 
discovered.  The  spiral  nebulae  may  be  entirely  within 
the  limits  of  this  system  or  they  may  be  even  more 
distant  than  the  globular  clusters  for  their  distances 
are  not  known  as  yet. 

There  is  a  possibility  that  our  sidereal  universe,  vast 
as  it  is  known  to  be,  may  be  but  a  unit  in  some  still 
greater  unit  and  that  other  similar  systems  lie  beyond 
the  reach  of  existing  telescopes  at  unimaginable 
distances. 

The  mind  of  man  is  overwhelmed  by  the  thought 
of  sidereal  systems  as  vast  as  our  own  lying  far  beyond 


ASTRONOMICAL  DISTANCES  249 

his  ken.  Whether  or  not  such  external  systems  do 
exist  and  are  with  our  own  sidereal  system  units  in 
some  still  vaster  creation  we  cannot  know. 

So  vast,  indeed,  is  this  one  visible  universe  of  ours 
that  the  mind  of  man,  accustomed  to  earthly  standards, 
cannot  comprehend  its  magnitude  or  the  infinitesimal 
size  of  our  whole  solar  system  compared  to  it. 


XXXI 

SOME  ASTRONOMICAL  FACTS  WORTH 
REMEMBERING 

KEPLER'S  Three  Laws  of  Planetary  Motion: 
I.    The  planets  move  in  ellipses  with  the  sun  at 
one  focus. 

II.  The  radius  vector  of  a  planet  (line  adjoining  sun 
and  planet)  sweeps  over  equal  areas  in  equal  times. 

III.  The  square  of  the  time  of  revolution  (the  year) 
of  each  planet  is  proportional  to  the  cube  of  its  mean 

distance  from  the  sun. 

*  *  *  *  * 

Sir  Isaac  Newton  discovered  that  the  law  of  gravita- 
tion extends  to  the  stars.  That  is,  every  mass  in  the 
universe  attracts  every  other  mass  with  an  attraction 
directly  proportional  to  the  product  of  the  masses  and 
inversely  proportional  to  the  square  of  the  distances  be- 
tween them. 

*  *  *  *  * 

Ocean  tides  are  caused  by  the  difference  between  the 
attraction  of  the  sun  and  moon  for  the  main  body  of 
the  earth  and  their  attraction  for  different  particles  of 
the  earth's  surface.  The  tide-raising  force  of  the  dis- 
turbing body  is  proportional  to  its  mass  and  inversely 
proportional  to  the  cube  of  its  distance.  The  tides 

250 


ASTRONOMICAL  FACTS  251 

produced  by  the  sun  are,  therefore,  only  two-fifths  as 

great  as  the  tides  produced  by  the  moon. 

*  *  *  *  * 

The  celestial  sphere  is  an  imaginary  sphere  of  infinite 
radius,  with  the  earth  at  its  center,  upon  which  the 
celestial  bodies  are  considered  to  be  projected  for  con- 
venience in  determining  their  positions  with  respect  to 
fixed  points  of  reference  in  the  heavens. 

The  north  and  south  poles  of  the  heavens  are  the 
points  on  the  celestial  sphere  directly  above  the  north 
and  south  poles  of  the  earth. 

The  celestial  equator  is  the  great  circle  in  which  the 
plane  of  the  earth's  equator  intersects  the  celestial 
sphere.  It  passes  through  the  east  and  west  points  of 
the  horizon  and  through  the  zenith — or  point  directly 
overhead — at  the  earth's  equator. 

The  ecliptic  is  the  great  circle  in  which  the  plane 
of  the  earth's  orbit  intersects  the  celestial  sphere.  The 
celestial  equator  and  the  ecliptic  are  inclined  to  each 
other  at  an  angle  of  23J/>°,  which  is  called  the  obliquity 
of  the  ecliptic.  The  two  points  in  which  the  celestial 
equator  and  the  ecliptic  intersect  are  called  respectively 
the  vernal  equinox  and  the  autumnal  equinox. 

The  vernal  equinox  is  an  important  point  of  reference 
on  the  celestial  sphere. 

As  the  position  of  a  point  on  the  earth's  surface  is 
determined  by  its  longitude  and  latitude  sx>  the  posi- 
tion of  an  object  on  the  celestial  sphere — star,  sun, 
planet — is  determined  by  its  Right  Ascension  and 
Declination. 


252      ASTRONOMY  FOR  YOUNG  FOLKS 

The  Declination  of  a  celestial  object  is  its  distance 
north  or  south  of  the  celestial  equator,  measured  in 
degrees,  minutes  and  seconds  of  arc,  on  a  great  circle 
of  the  celestial  sphere  passing  through  the  object  and 
north  and  south  poles  of  the  heavens.  These  great 
circles  are  called  hour  circles  and  they  correspond  to 
the  meridians  or  circles  of  longitude  on  the  earth's 
surface.  The  declination  of  an  object  in  the  heavens 
corresponds  to  the  latitude  of  a  point  on  the  earth's 
surface.  The  Right  Ascension  of  a  point  on  the 
celestial  sphere  corresponds  to  the  longitude  of  a  point 
on  the  earth's  surface.  It  is  measured — as  longitude  is 
measured — in  degrees,  minutes  and  seconds  of  arc  or  in 
hours,  minutes  and  seconds  of  time — eastward  along 
the  celestial  equator  from  the  hour  circle  passing 
through  the  vernal  equinox  to  the  foot  of  the  hour 
circle  passing  through  the  object.  The  hour  circle 
passing  through  the  vernal  equinox  is  the  zero  meridian 
for  the  celestial  sphere  just  as  the  meridian  of 

Greenwich  is  the  zero  meridian  on  the  earth's  surface. 

*  *  *  *  * 

The  mean  distance  of  the  earth  from  the  sun  is 
92,900,000  miles  and  is  called  the  astronomical  unit. 

The  sun  with  its  satellites  advances  through  the 
universe  at  the  rate  of  4  astronomical  units  in  a  year 
or  approximately  one  million  miles  a  day. 

The  parallax  of  a  star  is  the  angle  at  the  star  sub- 
tended by  the  radius  of  the  earth's  orbit,  92,900,000 
miles,  or  the  astronomical  unit.  It  is,  in  other  words, 
the  angular  distance  between  the  earth  and  sun  as 


ASTRONOMICAL  FACTS  253 

viewed  from  the  star.  The  larger  the  parallax  the 
nearer  the  star.  The  largest  known  stellar  parallax  is 
that  of  Alpha  Centauri  and  its  value  is  0".75. 

The  light-year  is  the  distance  that  light  travels  in  one 
year.  It  is  equal  to  about  63,000  astronomical  units  or 
nearly  six  trillion  (6,000,000,000,000)  miles.  The 
velocity  of  light  is  186,000  miles  per  second. 

The  parsec  is  equal  to  3.26  light-years.  It  is  the 
distance  of  a  star  that  has  a  parallax  of  one  second 
of  arc. 

The  apparent  magnitude  of  a  star  is  its  apparent 
brightness  estimated  on  a  scale  in  which  a  difference 
of  one  magnitude  corresponds  to  a  difference  in  bright- 
ness of  2.51,  or  the  fifth  root  of  one  hundred.  A 
difference  of  five  magnitudes  corresponds  to  a  difference 
one  one  hundredfold  in  brightness,  of  ten  magnitudes 
to  ten  thousandfold  in  brightness.  In  exact  measure- 
ments on  this  scale  magnitudes  are  estimated  to  tenths. 

Stars  that  are  one  magnitude  brighter  than  stars 
of  the  standard  first  magnitude  are  of  the  zero  magni- 
tude and  stars  still  brighter  are  of  negative  magnitudes. 

Sirius  is  a  star  of  the  — 1.6  magnitude.  Jupiter 
at  opposition  is  of  — 2.0  magnitude  and  Venus  at 
greatest  brilliancy  of  — 4.0  magnitude.  The  sun  on 
this  scale  of  comparative  brightness  is  of  the  — 26.7 
apparent  magnitude.  The  faintest  stars  visible  in  the 
most  powerful  telescope  in  the  world — the  101-inch 
Mt.  Wilson  Hooker  telescope — are  of  the  twentieth 
magnitude. 

The   absolute  magnitude   of   a  star   is   its   apparent 


254      ASTRONOMY  FOR  YOUNG  FOLKS 

magnitude  at  the  standard  distance  of  ten  parsecs  or 
32.6  light  years.  The  absolute  magnitude  of  the  sun 
is  five.  That  is,  the  sun  would  be  a  fifth-magnitude 
star  at  the  standard  distance  of  32.6  light-years.  The 
absolute  magnitudes  of  stars  indicate  how  bright  they 
would  be  relatively  if  they  were  all  at  the  same  stand- 
ard distance.  Apparent  magnitudes  indicate  how 
bright  the  stars  appear  to  be  at  their  true  distances. 
***** 

The  mean  distance  of  the  moon  from  the  earth  is 
approximately  240,000  miles  or  sixty  times  the  earth's 
radius. 

The  sun  is  four  hundred  times  farther  away  than 
the  moon  and  its  diameter  is  about  four  hundred  times 
greater  than  the  moon's  diameter. 

The  nearest  star  is  about  275,000  times  more  distant 
than  the  sun,  and  the  most  distant  known  object,  the 
.globular  star  cluster,  N.G.C.  7106,  is  about  fourteen 
billion  times  more  distant  than  the  sun. 

The  earth  is  a  spheroid  flattened  at  the  poles  and  its 
polar  diameter  is  about  twenty-seven  miles  shorter  than 
its  equatorial  diameter.  An  object  weighs  less  at  the 
poles  than  at  the  equator. 

The  earth's  interior  is  as  rigid  as  steel  and  probably 
consists  of  a  core  of  magnetic  iron  surrounded  by  an 
outer  stony  shell. 

Eclipses  of  the  sun  occur  when  the  moon  passes  be- 
tween the  earth  and  sun.  They  can  only  occur  at  the 
time  of  new  moon.  There  must  be  at  least  two  solar 
eclipses  every  year  separated  by  an  interval  of  six 


ASTRONOMICAL  FACTS  255 

months  and  there  may  be  as  many  as  five  solar  eclipses 
in  a  year.  Eclipses  of  the  moon  occur  when  the  earth 
comes  between  the  sun  and  moon,  and  the  moon  passes 
into  the  earth's  shadow.  Eclipses  of  the  moon  can 
only  occur  at  full  moon.  There  may  or  may  not  be 
eclipses  of  the  moon  every  year.  The  greatest  number 
of  eclipses  than  can  occur  in  any  one  year,  solar  and 
lunar  combined,  is  seven  and  the  least  number  is  two 
and  in  that  case  they  are  both  solar  eclipses. 

The  sun  is  a  yellow,  dwarf  star  of  a  density  of  one 
and  one- fourth  that  of  water  and  with  a  surface  tem- 
perature of  about  12,000°  F.  except  in  sunspot  regions 
where  the  temperature  is  about  6,000°  F.  It  is  prob- 
ably gaseous  throughout. 

The  sun,  as  well  as  the  planets,  rotates  on  its  axis 
and  different  portions  of  the  surface  rotate  at  slightly 
different  rates.  The  average  period  of  the  rotation  of 
the  sun  on  its  axis  is  about  twenty-six  days. 

The  sun  is  a  variable  star  with  a  twofold  variation. 
One  is  of  long  period  during  the  eleven-year  sunspot 
cycle  with  a  range  of  from  three  to  five  per  cent.  The 
other  is  a  short  irregular  variation  with  a  period  of  a 
few  days,  weeks  or  months  and  a  range  of  from  three 
to  ten  per  cent. 

Sunspots  are  soiar  cyclones  and  appear  black  only 
by  contrast  with  their  hotter  and  brighter  surroundings. 
They  come  in  eleven-year  cycles  (approximately)  with 
periods  of  maximum  and  minimum  appearance. 

The  brightness  and  blue  color  of  the  sky  is  due  to 
the  scattering  of  sunlight  by  the  molecules  of  oxygen 


256      ASTRONOMY  FOR  YOUNG  FOLKS 

and  nitrogen  in  the  earth's  upper  atmosphere.  If 
there  were  no  atmosphere  the  skies  would  appear  black 
except  in  the  direction  of  the  heavenly  bodies,  which 
would  be  visible  by  day  as  well  as  by  night. 

The  solar  corona  is  the  rare  outer  envelope  of  the  sun 
and  it  is  visible  only  during  a  total  eclipse  of  the  sun. 
It  is  partly  of  an  electrical  nature  and  it  varies  in  form 
during  the  sunspot  cycle.  It  often  extends  to  a  dis- 
tance of  several  solar  diameters  on  either  side  of 
the  sun. 

The  warmth  and  the  habitability  of  the  earth's  sur- 
face is  due  to  the  presence  of  water-vapor  and  carbon- 
dioxide  in  the  atmosphere.  Without  these  substances 
in  the  atmosphere  life  on  the  earth's  surface  would  be 
impossible. 

Half  of  the  earth's  atmosphere  and  all  clouds  lie 
within  seven  miles  of  the  earth's  surface,  and  at  high 
elevations  above  the  earth  the  temperature  is  many 
degrees  below  zero. 

The  temperature  of  space  approaches  the  absolute 
zero  of  —459°  F. 

The  only  planets  in  the  solar  system  with  the  excep- 
tion of  the  earth  that  might  possibly  support  life  are 
Venus  and  Mars. 

Stars  shine  by  <their  own  light  but  planets  shine  only 

by  reflected  light  from  the  sun. 

***** 

If  the  earth  were  represented  by  a  six-inch  school 
globe  the  sun  would  be  on  the  same  scale  a  globe  fifty- 
four  feet  in  diameter.  Mercury  would  be  a  small  ball 


ASTRONOMICAL  FACTS  257 

two  and  a  third  inches  in  diameter.  Venus  would  be 
another  six-inch  globe.  Mars  would  be  a  ball  about 
the  size  of  a  baseball,  three  and  a  fifth  inches  in  diam- 
eter. The  moon  would  be  about  the  size  of  a  golf  ball, 
one  and  a  half  inches  in  diameter.  The  largest  asteroids 
would  be  the  size  of  marbles.  Average-sized  asteroids 
would  be  the  size  of  shot  and  the  smallest  would  be 
merely  grains  of  sand. 

Jupiter  would  be  a  huge  globe  standing  as  tall  as  a 
man  five  feet  six  inches  in  height.  Saturn  would  be 
a  smaller  globe  four  and  a  half  feet  in  diameter  and  its 
ring  system  would  extend  to  a  distance  of  five  and  a 
half  feet  on  either  side  of  the  globe.  Uranus  would  be 
represented  by  a  globe  almost  exactly  two  feet  in 
diameter  and  Neptune  would  be  a  slightly  larger  globe 
with  a  diameter  of  two  feet  two  and  a  half  inches. 

The  satellites  of  the  outer  planets  would  range  in 
size  from  tennis  and  golf  balls  for  the  largest,  to  mar- 
bles for  the  smaller  and  grains  of  sand  for  the  smallest. 

On  the  same  scale  of  measurement  the  distance  of 
the  six-inch  globe  of  the  earth  from  the  fifty- four  foot 
globe  representing  'the  sun  would  be  one  and  one-tenth 
miles.  The  moon  would  be  placed  fifteen  feet  from  the 
earth-globe  and  the  diameter  of  the  solar  system  on  the 
same  scale  measured  across  the  orbit  of  Neptune  would 
be  sixty-six  miles.  The  nearest  star  on  this  scale  would 

be  three  hundred  thousand  miles  away. 

***** 

If  the  distance  from  the  earth  to  the  sun  is  taken 
as  one  inch  so  that  the  scale  of  the  universe  is  reduced 


258      ASTRONOMY  FOR  YOUNG  FOLKS 

six  trillion  times,  the  diameter  of  the  solar  system 
across  Neptune's  orbit  is  five  feet  and  the  distance  of 
one  light-year  comes  out  almost  exactly  equal  to  one 
mile.  The  nearest  star  to  the  five- foot  solar  system 
would  be  four  and  a  third  miles  away ;  the  most  distant 
known  object  would  be  two  hundred  and  twenty  thou- 
sand miles  away,  and  the  extent  of  the  visible  universe 
would  be  three  hundred  thousand  miles.  On  the 
same  scale  the  diameter  of  our  sun  would  be  about 
one  hundredth  of  an  inch  and  the  diameters  of  the 
giant  stars  Antares  and  Betelgeuze  would  be  four 
inches  and  two  and  three- fourth  inches  respectively. 
To  see  the  earth  we  would  need  a  microscope. 


TABLES 


TABLES 


261 


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262 


TABLES 


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TABLES 


263 


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264  TABLES 

TABLE  IV 

A  LIST  OF  THE  PRINCIPAL  CONSTELLATIONS 

1.    VISIBLE  IN  40°  NORTH  LATITUDE 


Name 

Chief  Star  or  Noted  Object 

On  Meridian 
9  P.  M. 

Passes 
Overhead 
in  Latitude* 
(Degrees) 

Andromeda  

Great  Nebula 

35  N 

October 

5  S 

Altair 

0° 

Aries 

20  N 

Auriga           . 

Capella 

40  N 

Bootes     

Arcturus 

30  N 

Cancer        

Praesepe 

March 

20  N 

Canes  Venatici  

Cor  Caroli 

June 

40  N 

Canis  Major  

Sirius 

March 

20  S. 

Canis  Minor  

Procyon 

March 

ION. 

October 

15  S. 

60  N 

Ceoheus 

70  N 

Cetus       

Mira 

December 

5S. 

Columba  

February 

35  S. 

Coma  Berenices  

May 

25  N. 

Corona  Borealis  

Alphacca 

July 

SON. 

May 

20  S 

Crater 

May 

15  S. 

Cvsnus 

Deneb 

September 

40  N. 

Delphinus  

Most  distant  globular  cluster 

September 

15  N. 

Draco  

Alpha 

August 

65  N. 

Eridanus  

Achernar 

January 

10°  N.  to  60°  S. 

Pollux 

March 

25  N. 

Hercules      .     . 

Great  Cluster 

July 

30  N. 

Hydra  

April 

20  S. 

Leo  

Regulus 

April 

15  N. 

February 

20  S. 

Libra 

June 

15  S. 

Lynx 

April 

40  N. 

Lyr  a          

Vega 

August 

40  N. 

Optiinchus  

July 

10  S. 

Orion  

Great  Nebula 

February 

0° 

Piscis  Australis  
Pegasus 

Fomalhaut 

October 
November 

308. 
20  N. 

Algol 

January 

60  N. 

Pisces 

December 

6N. 

Saeitta 

September 

20  N. 

Sagittarius  

August 

308. 

t/ 


TABLES 

TABLE  IV 

A  LIST  OF  THE  PRINCIPAL  CONSTELLATIONS 

(Continued) 
1.    VISIBLE  IN  40°  NORTH  LATITUDE 


265 


Name 

Chief  Star  or  Noted  Object 

On  Meridian 
9  P.  M. 

Passes 
Overhead 
in  Latitude* 
(Degrees) 

Scorpio 

An  tares 

July 

SOS. 

Serpens                   .    . 

July 

20°  N.  to  15°  S. 

Taurus 

Pleiades 

January 

20  N. 

35  N 

Ursa  Major  

Mizar 

May 

65  N. 

Ursa  Minor  

Polaris 

85  N. 

Virgo  

Spica 

June 

0° 

2.     INVISIBLE  IN  40°  NORTH  LATITUDE 


Apus.  .                  .... 

July 

75  S. 

Ara.            

July 

65  S. 

Argo  Navis  

Canopus 

March 

50  S. 

1      Carina 

March 

60  S 

2      PUDDIS 

March 

45  S 

3     Vela 

March 

50  S 

Centaurus 

Alpha  Centauri 

June 

50  S. 

Crux  (Southern  Cross) 
Dorado  

Alpha  Crucis 
Gt.  Magellanic  Cloud 

June 
February 

60S. 

58  S. 

Grus 

October 

45  S 

Hydrus 

Lesser  Mag.  Cloud 

70S 

Indus 

September 

55  S. 

Lupus  .  . 

June 

40  S. 

Musca  

June 

70S. 

Octans 

85  S. 

Pavo 

October 

65  S 

Phoenix 

November 

45  S. 

Telescopium 

July 

48  S. 

Triangulum  Australe  .  . 

July 

65  S. 

Tucana  

Great  Cluster 

November 

60  S. 

Volans 

March 

75  s 

The  approximate  position  of  the  center  of  the  constellation. 


266 


TABLES 


TABLE  V 

PRONUNCIATIONS  AND  MEANINGS  OF  NAMES  OF  STARS  AND 
CONSTELLATIONS 

1.    STARS 


Name 

Pronunciation 

Meaning 

Achernar  

a-ke'r-nar 

End-of  -the-  River 

Aldebaran  

al-de  'b-ar-an 

The  Hindmost 

Altair 

al-ta'r 

Antares 

Rival  of  Ares  (Mars) 

Arcturus.'  

ark-t'u-rus 

Bellatrix  

bel-la'trix 

The  Female  Warrior 

Betelgeu  ze  

be't-el-gerz  or  be't-el-gez 

The  Arm-  Pit 

Caoella 

Little  She-Goat 

Denebola       ..... 

de-ne'b-o-la 

The  Lion's  Tail 

Fomalhaut  

fo'-mal-o 

The  Fish's  Mouth 

The  Rainy  Ones 

Pleiades 

Pollux          .... 

po'l-lux 

Praesepe 

pre-se'-pe 

The  Bee-hive 

Procyon  

pro-si'-on 

Precursor  of  the  Dog 

Regulus  ... 

reg'-u-lua 

The  Ruler 

Rigel 

ri'-gel  or  ri-jel 

Sirius  

sirM-us 

The  Sparkling  One 

Spica 

spi'-ka 

The  Ear  of  Wheat 

Vega  

ve'-ga 

2.     CONSTELLATIONS 


an-d  'rom-e-da 

The  Woman  Chained 

Aquarius  

a-kw'a-ri-us 

The  Water-bearer 

a'k-wi-la 

The  Eagle 

Ara 

a'-ra 

The  Altar 

ft'r-go-n'a-vis 

The  Ship  Argo 

Aries      

a'-rez 

The  Ram 

Auriga  

aw-ri'-ga 

The  Charioteer 

Bootes  

bo-o'-tez 

The  Herdsman 

ca'n-ser 

The  Crab 

Canes  Venatici 

ca'-nez  ven-a't-i-si 

The  Hunting  Dogs 

Canis  Major  

ca'-nis  ma'jor 

The  Greater  Dog 

Canis  Minor  

ca-'nis  mi'nor 

The  Lesser  Dog 

ca'p-ri-ko'r-nus 

The  Goat 

Centaurus         .          ... 

cen-ta'w-rus 

The  Centaur 

se-fe-us 

Cetus  

s'e-tus 

The  Whale 

col-u'm-ba 

The  Dove 

TABLES 


267 


TABLE  V 

PRONUNCIATIONS  AND  MEANINGS  OF  NAMES  OF  STARS  AND 
CONSTELLATIONS 

2.    CONSTELLATIONS 
(Continued) 


Name 

Pronunciation 

Meaning 

Berenice's  Hair 

co-ro'-na  bo-re-a'-lis 

The  Northern  Crown 

c6'r-vus 

The  Crow 

Crater                             .    . 

cr'a-ter 

The  Cup 

Cru  x                               

kru'x 

The  Cross 

si'g-nus 

The  Swan 

del-fi'-nus 

The  Dolphin 

Dorado               

ddr-a'-do 

The  Gold-fish 

Draco    .       

dra'-co 

The  Dragon 

e-ri'd-a-nus 

The  River  Eridanus 

jem'-i-ni 

The  Twins 

gru's 

The  Crane 

Hercules 

her-ku-lez 

hi'dra 

The  Water-snake 

Hydrus                  

hi'-drus 

The  Serpent 

i'nd-us 

The  Indian 

Leo               

le'-o 

The  Lion 

le'-pus 

The  Hare 

li'-bra 

The  Scales 

lu'-pus 

The  Wolf 

The  Fox 

li'-ra 

The  Lyre 

Musca          

mus'-ca 

The  Fly 

Octans           

o'ct-ans 

The  Octant 

o'-fi-u'-kus 

The  Serpent-holder 

Orion  

o-ri'-on 

The  Warrior 

pa'-vo 

The  Peacock 

fe'-nix 

Piscis  Australis    

pi's-sis  aus-tra'-lis 

The  Southern  Fish 

Pegasus     

peg'-a-sus 

The  Winged  Horse 

pe'r-se-us  or  per-sus 

Pisces  

pi's-sez 

The  Fishes 

Sagitta 

sa-ji't-ta 

The  Arrow 

sa-jit-ta'-ri-us 

The  Archer 

sk6'r-pi-o 

The  Scorpion 

Serpens  

ser-pens 

The  Serpent 

tau-rus 

The  Bull 

tel-es-cop'-i-um 

The  Telescope 

tri-a'n-gu-lum 

The  Triangle 

Tucana                         •,  .  .  .  •,  •  •, 

tu'c-an-a 

The  Toucan 

Ursa  Major           

u'r-sa  ma'-jor 

The  Greater  Bear 

Ursa  Minor  

u'r-sa  mi'-nor 

The  Lesser  Bear 

Vireo  .. 

ve'r-go 

The  Maiden 

Volans  

vol-ans 

The  Flying  Fish 

14  DAY  USE 

RETURN  TO  DESK  FROM  WHICH  BORROWED 

LOAN  DEPT. 

This  book  is  due  on  the  last  date  stamped  below,  or 

on  the  date  to  which  renewed. 
Renewed  books  are  subject  to  immediate  recall. 


LD  21A-50m-ll,'62 
(D3279slO)476B 


General  Library 

University  of  California 

Berkeley 


YC  22212 


UNIVERSITY  OF  CALIFORNIA  LIBRARY 


